Update Newtonsoft.Json project to use nullable reference types (#1950)

Build/build.ps1

@@ -14,7 +14,7 @@
   $workingName = if ($workingName) {$workingName} else {"Working"}
   $assemblyVersion = if ($assemblyVersion) {$assemblyVersion} else {$majorVersion + '.0.0'}
   $netCliChannel = "2.0"
-  $netCliVersion = "2.2.105"
+  $netCliVersion = "3.0.100-preview8-013317"
   $nugetUrl = "https://dist.nuget.org/win-x86-commandline/latest/nuget.exe"
 
   $baseDir  = resolve-path ..

Src/Newtonsoft.Json.Tests/Converters/XmlNodeConverterTest.cs

@@ -160,6 +160,14 @@ namespace Newtonsoft.Json.Tests.Converters
             Assert.AreEqual("<Time>0001-01-01T00:00:00</Time>", xmlNode.OuterXml);
         }
 
+        [Test]
+        public void XmlNode_Null()
+        {
+            string json = JsonConvert.SerializeXmlNode(null);
+
+            Assert.AreEqual("null", json);
+        }
+
         [Test]
         public void XmlNode_Roundtrip_PropertyNameWithColon()
         {
@@ -305,6 +313,14 @@ namespace Newtonsoft.Json.Tests.Converters
         }
 
 #if !NET20
+        [Test]
+        public void XNode_Null()
+        {
+            string json = JsonConvert.SerializeXNode(null);
+
+            Assert.AreEqual("null", json);
+        }
+
         [Test]
         public void XNode_UnescapeTextContent()
         {

Src/Newtonsoft.Json.Tests/JsonConvertTest.cs

@@ -1563,14 +1563,15 @@ namespace Newtonsoft.Json.Tests
             public int Overload { get; set; }
         }
 
-        [Test]
-        public void JsonConverterConstructor_OverloadsWithBaseTypes()
-        {
-            OverloadWithBaseType value = new OverloadWithBaseType();
-            string json = JsonConvert.SerializeObject(value);
+        //[Test]
+        //[Ignore("https://github.com/dotnet/roslyn/issues/36974")]
+        //public void JsonConverterConstructor_OverloadsWithBaseTypes()
+        //{
+        //    OverloadWithBaseType value = new OverloadWithBaseType();
+        //    string json = JsonConvert.SerializeObject(value);
 
-            Assert.AreEqual("{\"Overload\":\"IList<string>\"}", json);
-        }
+        //    Assert.AreEqual("{\"Overload\":\"IList<string>\"}", json);
+        //}
 
 
         [Test]

Src/Newtonsoft.Json.Tests/Linq/JTokenTests.cs

@@ -242,6 +242,26 @@ namespace Newtonsoft.Json.Tests.Linq
             Assert.AreEqual(2, a[2].BeforeSelf().Count());
         }
 
+        [Test]
+        public void BeforeSelf_NoParent_ReturnEmpty()
+        {
+            JObject o = new JObject();
+
+            List<JToken> result = o.BeforeSelf().ToList();
+            Assert.AreEqual(0, result.Count);
+        }
+
+        [Test]
+        public void BeforeSelf_OnlyChild_ReturnEmpty()
+        {
+            JArray a = new JArray();
+            JObject o = new JObject();
+            a.Add(o);
+
+            List<JToken> result = o.BeforeSelf().ToList();
+            Assert.AreEqual(0, result.Count);
+        }
+
         [Test]
         public void Casting()
         {

Src/Newtonsoft.Json.Tests/Linq/JsonPath/QueryExpressionTests.cs

@@ -45,33 +45,12 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void AndExpressionTest()
         {
-            CompositeExpression compositeExpression = new CompositeExpression
+            CompositeExpression compositeExpression = new CompositeExpression(QueryOperator.And)
             {
-                Operator = QueryOperator.And,
                 Expressions = new List<QueryExpression>
                 {
-                    new BooleanQueryExpression
-                    {
-                        Operator = QueryOperator.Exists,
-                        Left = new List<PathFilter>
-                        {
-                            new FieldFilter
-                            {
-                                Name = "FirstName"
-                            }
-                        }
-                    },
-                    new BooleanQueryExpression
-                    {
-                        Operator = QueryOperator.Exists,
-                        Left = new List<PathFilter>
-                        {
-                            new FieldFilter
-                            {
-                                Name = "LastName"
-                            }
-                        }
-                    }
+                    new BooleanQueryExpression(QueryOperator.Exists, new List<PathFilter> { new FieldFilter("FirstName") }, null),
+                    new BooleanQueryExpression(QueryOperator.Exists, new List<PathFilter> { new FieldFilter("LastName") }, null)
                 }
             };
 
@@ -103,33 +82,12 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void OrExpressionTest()
         {
-            CompositeExpression compositeExpression = new CompositeExpression
+            CompositeExpression compositeExpression = new CompositeExpression(QueryOperator.Or)
             {
-                Operator = QueryOperator.Or,
                 Expressions = new List<QueryExpression>
                 {
-                    new BooleanQueryExpression
-                    {
-                        Operator = QueryOperator.Exists,
-                        Left = new List<PathFilter>
-                        {
-                            new FieldFilter
-                            {
-                                Name = "FirstName"
-                            }
-                        }
-                    },
-                    new BooleanQueryExpression
-                    {
-                        Operator = QueryOperator.Exists,
-                        Left = new List<PathFilter>
-                        {
-                            new FieldFilter
-                            {
-                                Name = "LastName"
-                            }
-                        }
-                    }
+                    new BooleanQueryExpression(QueryOperator.Exists, new List<PathFilter> { new FieldFilter("FirstName") }, null),
+                    new BooleanQueryExpression(QueryOperator.Exists, new List<PathFilter> { new FieldFilter("LastName") }, null)
                 }
             };
 
@@ -161,30 +119,14 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void BooleanExpressionTest_RegexEqualsOperator()
         {
-            BooleanQueryExpression e1 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.RegexEquals,
-                Right = new JValue("/foo.*d/"),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e1 = new BooleanQueryExpression(QueryOperator.RegexEquals, new List<PathFilter> { new ArrayIndexFilter() }, new JValue("/foo.*d/"));
 
             Assert.IsTrue(e1.IsMatch(null, new JArray("food")));
             Assert.IsTrue(e1.IsMatch(null, new JArray("fooood and drink")));
             Assert.IsFalse(e1.IsMatch(null, new JArray("FOOD")));
             Assert.IsFalse(e1.IsMatch(null, new JArray("foo", "foog", "good")));
 
-            BooleanQueryExpression e2 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.RegexEquals,
-                Right = new JValue("/Foo.*d/i"),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e2 = new BooleanQueryExpression(QueryOperator.RegexEquals, new List<PathFilter> { new ArrayIndexFilter() }, new JValue("/Foo.*d/i"));
 
             Assert.IsTrue(e2.IsMatch(null, new JArray("food")));
             Assert.IsTrue(e2.IsMatch(null, new JArray("fooood and drink")));
@@ -195,28 +137,12 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void BooleanExpressionTest_RegexEqualsOperator_CornerCase()
         {
-            BooleanQueryExpression e1 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.RegexEquals,
-                Right = new JValue("/// comment/"),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e1 = new BooleanQueryExpression(QueryOperator.RegexEquals, new List<PathFilter> { new ArrayIndexFilter() }, new JValue("/// comment/"));
 
             Assert.IsTrue(e1.IsMatch(null, new JArray("// comment")));
             Assert.IsFalse(e1.IsMatch(null, new JArray("//comment", "/ comment")));
 
-            BooleanQueryExpression e2 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.RegexEquals,
-                Right = new JValue("/<tag>.*</tag>/i"),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e2 = new BooleanQueryExpression(QueryOperator.RegexEquals, new List<PathFilter> { new ArrayIndexFilter() }, new JValue("/<tag>.*</tag>/i"));
 
             Assert.IsTrue(e2.IsMatch(null, new JArray("<Tag>Test</Tag>", "")));
             Assert.IsFalse(e2.IsMatch(null, new JArray("<tag>Test<tag>")));
@@ -225,15 +151,7 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void BooleanExpressionTest()
         {
-            BooleanQueryExpression e1 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.LessThan,
-                Right = new JValue(3),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e1 = new BooleanQueryExpression(QueryOperator.LessThan, new List<PathFilter> { new ArrayIndexFilter() }, new JValue(3));
 
             Assert.IsTrue(e1.IsMatch(null, new JArray(1, 2, 3, 4, 5)));
             Assert.IsTrue(e1.IsMatch(null, new JArray(2, 3, 4, 5)));
@@ -241,15 +159,7 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
             Assert.IsFalse(e1.IsMatch(null, new JArray(4, 5)));
             Assert.IsFalse(e1.IsMatch(null, new JArray("11", 5)));
 
-            BooleanQueryExpression e2 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.LessThanOrEquals,
-                Right = new JValue(3),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e2 = new BooleanQueryExpression(QueryOperator.LessThanOrEquals, new List<PathFilter> { new ArrayIndexFilter() }, new JValue(3));
 
             Assert.IsTrue(e2.IsMatch(null, new JArray(1, 2, 3, 4, 5)));
             Assert.IsTrue(e2.IsMatch(null, new JArray(2, 3, 4, 5)));
@@ -261,15 +171,7 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void BooleanExpressionTest_GreaterThanOperator()
         {
-            BooleanQueryExpression e1 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.GreaterThan,
-                Right = new JValue(3),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e1 = new BooleanQueryExpression(QueryOperator.GreaterThan, new List<PathFilter> { new ArrayIndexFilter() }, new JValue(3));
 
             Assert.IsTrue(e1.IsMatch(null, new JArray("2", "26")));
             Assert.IsTrue(e1.IsMatch(null, new JArray(2, 26)));
@@ -280,15 +182,7 @@ namespace Newtonsoft.Json.Tests.Linq.JsonPath
         [Test]
         public void BooleanExpressionTest_GreaterThanOrEqualsOperator()
         {
-            BooleanQueryExpression e1 = new BooleanQueryExpression
-            {
-                Operator = QueryOperator.GreaterThanOrEquals,
-                Right = new JValue(3),
-                Left = new List<PathFilter>
-                {
-                    new ArrayIndexFilter()
-                }
-            };
+            BooleanQueryExpression e1 = new BooleanQueryExpression(QueryOperator.GreaterThanOrEquals, new List<PathFilter> { new ArrayIndexFilter() }, new JValue(3));
 
             Assert.IsTrue(e1.IsMatch(null, new JArray("2", "26")));
             Assert.IsTrue(e1.IsMatch(null, new JArray(2, 26)));

Src/Newtonsoft.Json.Tests/Newtonsoft.Json.Tests.csproj

@@ -42,8 +42,8 @@
   </ItemGroup>
   
   <ItemGroup Condition="'$(TargetFramework)'=='net46'">
-    <PackageReference Include="NUnit" Version="3.10.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <PackageReference Include="Autofac" Version="4.6.2" />
     <PackageReference Include="BenchmarkDotNet" Version="0.10.10" />
     <PackageReference Include="FSharp.Core" Version="4.2.3" />    
@@ -66,8 +66,8 @@
   </PropertyGroup>
   
   <ItemGroup Condition="'$(TargetFramework)'=='net451'">
-    <PackageReference Include="NUnit" Version="3.10.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <PackageReference Include="Autofac" Version="4.6.2" />
     <PackageReference Include="FSharp.Core" Version="4.2.3" />
     <PackageReference Include="System.Buffers" Version="4.4.0" />
@@ -89,8 +89,8 @@
   </PropertyGroup>
   
   <ItemGroup Condition="'$(TargetFramework)'=='net452'">
-    <PackageReference Include="NUnit" Version="3.10.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <PackageReference Include="Autofac" Version="4.6.2" />
     <PackageReference Include="FSharp.Core" Version="4.2.3" />
     <PackageReference Include="System.Buffers" Version="4.4.0" />
@@ -112,8 +112,8 @@
   </PropertyGroup>
   
   <ItemGroup Condition="'$(TargetFramework)'=='net40'">
-    <PackageReference Include="NUnit" Version="3.10.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <PackageReference Include="System.ValueTuple" Version="4.4.0" />
     <Reference Include="Microsoft.CSharp" />
     <Reference Include="System.Web" />
@@ -129,8 +129,8 @@
   </PropertyGroup>
   
   <ItemGroup Condition="'$(TargetFramework)'=='net35'">
-    <PackageReference Include="NUnit" Version="3.10.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <Reference Include="System.Web" />
     <Reference Include="System.Data.Linq" />
     <Reference Include="System.Data.Entity" />
@@ -144,8 +144,8 @@
   </PropertyGroup>
   
   <ItemGroup Condition="'$(TargetFramework)'=='net20'">
-    <PackageReference Include="NUnit" Version="3.10.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
     <Reference Include="System.Web" />
   </ItemGroup>
   <PropertyGroup Condition="'$(TargetFramework)'=='net20'">

Src/Newtonsoft.Json.Tests/Serialization/ContractResolverTests.cs

@@ -24,6 +24,7 @@
 #endregion
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.IO;
@@ -727,6 +728,17 @@ namespace Newtonsoft.Json.Tests.Serialization
             }
         }
 
+        [Test]
+        public void NonGenericDictionary_KeyValueTypes()
+        {
+            DefaultContractResolver resolver = new DefaultContractResolver();
+
+            JsonDictionaryContract c = (JsonDictionaryContract)resolver.ResolveContract(typeof(IDictionary));
+
+            Assert.IsNull(c.DictionaryKeyType);
+            Assert.IsNull(c.DictionaryValueType);
+        }
+
         [Test]
         public void DefaultContractResolverIgnoreIsSpecifiedTrue()
         {

Src/Newtonsoft.Json.Tests/TestFixtureBase.cs

@@ -212,11 +212,9 @@ namespace Newtonsoft.Json.Tests
 
         public static string ResolvePath(string path)
         {
-#if !DNXCORE50
-            return Path.Combine(TestContext.CurrentContext.TestDirectory, path);
-#else
-            return path;
-#endif
+            var assemblyPath = Path.GetDirectoryName(typeof(TestFixtureBase).Assembly().Location);
+
+            return Path.Combine(assemblyPath, path);
         }
 
         protected string GetOffset(DateTime d, DateFormatHandling dateFormatHandling)

Src/Newtonsoft.Json/Bson/BsonBinaryType.cs

@@ -25,6 +25,8 @@
 
 using System;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     internal enum BsonBinaryType : byte

Src/Newtonsoft.Json/Bson/BsonBinaryWriter.cs

@@ -29,6 +29,8 @@ using System.IO;
 using System.Text;
 using Newtonsoft.Json.Utilities;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     internal class BsonBinaryWriter

Src/Newtonsoft.Json/Bson/BsonObjectId.cs

@@ -26,6 +26,8 @@
 using System;
 using Newtonsoft.Json.Utilities;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     /// <summary>

Src/Newtonsoft.Json/Bson/BsonReader.cs

@@ -32,6 +32,8 @@ using Newtonsoft.Json.Serialization;
 using Newtonsoft.Json.Utilities;
 using Newtonsoft.Json.Linq;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     /// <summary>

Src/Newtonsoft.Json/Bson/BsonToken.cs

@@ -26,6 +26,8 @@
 using System.Collections;
 using System.Collections.Generic;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     internal abstract class BsonToken

Src/Newtonsoft.Json/Bson/BsonType.cs

@@ -23,6 +23,8 @@
 // OTHER DEALINGS IN THE SOFTWARE.
 #endregion
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     internal enum BsonType : sbyte

Src/Newtonsoft.Json/Bson/BsonWriter.cs

@@ -35,6 +35,8 @@ using Newtonsoft.Json.Utilities;
 using Newtonsoft.Json.Linq;
 using System.Globalization;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Bson
 {
     /// <summary>

Src/Newtonsoft.Json/Converters/BinaryConverter.cs

@@ -51,7 +51,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -70,7 +70,7 @@ namespace Newtonsoft.Json.Converters
             if (value.GetType().FullName == BinaryTypeName)
             {
                 EnsureReflectionObject(value.GetType());
-                return (byte[])_reflectionObject.GetValue(value, BinaryToArrayName);
+                return (byte[])_reflectionObject.GetValue(value, BinaryToArrayName)!;
             }
 #endif
 #if HAVE_ADO_NET
@@ -101,7 +101,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -123,7 +123,7 @@ namespace Newtonsoft.Json.Converters
             {
                 // current token is already at base64 string
                 // unable to call ReadAsBytes so do it the old fashion way
-                string encodedData = reader.Value.ToString();
+                string encodedData = reader.Value!.ToString();
                 data = Convert.FromBase64String(encodedData);
             }
             else
@@ -140,7 +140,7 @@ namespace Newtonsoft.Json.Converters
             {
                 EnsureReflectionObject(t);
 
-                return _reflectionObject.Creator(data);
+                return _reflectionObject.Creator!(data);
             }
 #endif
 

Src/Newtonsoft.Json/Converters/BsonObjectIdConverter.cs

@@ -28,6 +28,8 @@ using Newtonsoft.Json.Bson;
 using System.Globalization;
 using Newtonsoft.Json.Utilities;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Converters
 {
     /// <summary>

Src/Newtonsoft.Json/Converters/CustomCreationConverter.cs

@@ -41,7 +41,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             throw new NotSupportedException("CustomCreationConverter should only be used while deserializing.");
         }
@@ -54,7 +54,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {

Src/Newtonsoft.Json/Converters/DataSetConverter.cs

@@ -41,7 +41,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -50,7 +50,7 @@ namespace Newtonsoft.Json.Converters
             }
 
             DataSet dataSet = (DataSet)value;
-            DefaultContractResolver resolver = serializer.ContractResolver as DefaultContractResolver;
+            DefaultContractResolver? resolver = serializer.ContractResolver as DefaultContractResolver;
 
             DataTableConverter converter = new DataTableConverter();
 
@@ -74,7 +74,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -92,10 +92,10 @@ namespace Newtonsoft.Json.Converters
 
             while (reader.TokenType == JsonToken.PropertyName)
             {
-                DataTable dt = ds.Tables[(string)reader.Value];
+                DataTable dt = ds.Tables[(string)reader.Value!];
                 bool exists = (dt != null);
 
-                dt = (DataTable)converter.ReadJson(reader, typeof(DataTable), dt, serializer);
+                dt = (DataTable)converter.ReadJson(reader, typeof(DataTable), dt, serializer)!;
 
                 if (!exists)
                 {

Src/Newtonsoft.Json/Converters/DataTableConverter.cs

@@ -45,7 +45,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -54,7 +54,7 @@ namespace Newtonsoft.Json.Converters
             }
 
             DataTable table = (DataTable)value;
-            DefaultContractResolver resolver = serializer.ContractResolver as DefaultContractResolver;
+            DefaultContractResolver? resolver = serializer.ContractResolver as DefaultContractResolver;
 
             writer.WriteStartArray();
 
@@ -87,7 +87,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -106,7 +106,7 @@ namespace Newtonsoft.Json.Converters
             // populate the name from the property name
             if (reader.TokenType == JsonToken.PropertyName)
             {
-                dt.TableName = (string)reader.Value;
+                dt.TableName = (string)reader.Value!;
 
                 reader.ReadAndAssert();
 
@@ -140,7 +140,7 @@ namespace Newtonsoft.Json.Converters
 
             while (reader.TokenType == JsonToken.PropertyName)
             {
-                string columnName = (string)reader.Value;
+                string columnName = (string)reader.Value!;
 
                 reader.ReadAndAssert();
 
@@ -177,7 +177,7 @@ namespace Newtonsoft.Json.Converters
                         reader.ReadAndAssert();
                     }
 
-                    List<object> o = new List<object>();
+                    List<object?> o = new List<object?>();
 
                     while (reader.TokenType != JsonToken.EndArray)
                     {
@@ -218,7 +218,7 @@ namespace Newtonsoft.Json.Converters
                 case JsonToken.String:
                 case JsonToken.Date:
                 case JsonToken.Bytes:
-                    return reader.ValueType;
+                    return reader.ValueType!;
                 case JsonToken.Null:
                 case JsonToken.Undefined:
                 case JsonToken.EndArray:

Src/Newtonsoft.Json/Converters/DiscriminatedUnionConverter.cs

@@ -48,17 +48,32 @@ namespace Newtonsoft.Json.Converters
         #region UnionDefinition
         internal class Union
         {
-            public List<UnionCase> Cases;
-            public FSharpFunction TagReader { get; set; }
+            public readonly FSharpFunction TagReader;
+            public readonly List<UnionCase> Cases;
+
+            public Union(FSharpFunction tagReader, List<UnionCase> cases)
+            {
+                TagReader = tagReader;
+                Cases = cases;
+            }
         }
 
         internal class UnionCase
         {
-            public int Tag;
-            public string Name;
-            public PropertyInfo[] Fields;
-            public FSharpFunction FieldReader;
-            public FSharpFunction Constructor;
+            public readonly int Tag;
+            public readonly string Name;
+            public readonly PropertyInfo[] Fields;
+            public readonly FSharpFunction FieldReader;
+            public readonly FSharpFunction Constructor;
+
+            public UnionCase(int tag, string name, PropertyInfo[] fields, FSharpFunction fieldReader, FSharpFunction constructor)
+            {
+                Tag = tag;
+                Name = name;
+                Fields = fields;
+                FieldReader = fieldReader;
+                Constructor = constructor;
+            }
         }
         #endregion
 
@@ -74,31 +89,28 @@ namespace Newtonsoft.Json.Converters
             // need to get declaring type to avoid duplicate Unions in cache
 
             // hacky but I can't find an API to get the declaring type without GetUnionCases
-            object[] cases = (object[])FSharpUtils.GetUnionCases(null, t, null);
+            object[] cases = (object[])FSharpUtils.GetUnionCases(null, t, null)!;
 
             object caseInfo = cases.First();
 
-            Type unionType = (Type)FSharpUtils.GetUnionCaseInfoDeclaringType(caseInfo);
+            Type unionType = (Type)FSharpUtils.GetUnionCaseInfoDeclaringType(caseInfo)!;
             return unionType;
         }
 
         private static Union CreateUnion(Type t)
         {
-            Union u = new Union();
+            Union u = new Union((FSharpFunction)FSharpUtils.PreComputeUnionTagReader(null, t, null), new List<UnionCase>());
 
-            u.TagReader = (FSharpFunction)FSharpUtils.PreComputeUnionTagReader(null, t, null);
-            u.Cases = new List<UnionCase>();
-
-            object[] cases = (object[])FSharpUtils.GetUnionCases(null, t, null);
+            object[] cases = (object[])FSharpUtils.GetUnionCases(null, t, null)!;
 
             foreach (object unionCaseInfo in cases)
             {
-                UnionCase unionCase = new UnionCase();
-                unionCase.Tag = (int)FSharpUtils.GetUnionCaseInfoTag(unionCaseInfo);
-                unionCase.Name = (string)FSharpUtils.GetUnionCaseInfoName(unionCaseInfo);
-                unionCase.Fields = (PropertyInfo[])FSharpUtils.GetUnionCaseInfoFields(unionCaseInfo);
-                unionCase.FieldReader = (FSharpFunction)FSharpUtils.PreComputeUnionReader(null, unionCaseInfo, null);
-                unionCase.Constructor = (FSharpFunction)FSharpUtils.PreComputeUnionConstructor(null, unionCaseInfo, null);
+                UnionCase unionCase = new UnionCase(
+                    (int)FSharpUtils.GetUnionCaseInfoTag(unionCaseInfo),
+                    (string)FSharpUtils.GetUnionCaseInfoName(unionCaseInfo),
+                    (PropertyInfo[])FSharpUtils.GetUnionCaseInfoFields(unionCaseInfo)!,
+                    (FSharpFunction)FSharpUtils.PreComputeUnionReader(null, unionCaseInfo, null),
+                    (FSharpFunction)FSharpUtils.PreComputeUnionConstructor(null, unionCaseInfo, null));
 
                 u.Cases.Add(unionCase);
             }
@@ -112,9 +124,15 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
-            DefaultContractResolver resolver = serializer.ContractResolver as DefaultContractResolver;
+            if (value == null)
+            {
+                writer.WriteNull();
+                return;
+            }
+
+            DefaultContractResolver? resolver = serializer.ContractResolver as DefaultContractResolver;
 
             Type unionType = UnionTypeLookupCache.Get(value.GetType());
             Union union = UnionCache.Get(unionType);
@@ -127,7 +145,7 @@ namespace Newtonsoft.Json.Converters
             writer.WriteValue(caseInfo.Name);
             if (caseInfo.Fields != null && caseInfo.Fields.Length > 0)
             {
-                object[] fields = (object[])caseInfo.FieldReader.Invoke(value);
+                object[] fields = (object[])caseInfo.FieldReader.Invoke(value)!;
 
                 writer.WritePropertyName((resolver != null) ? resolver.GetResolvedPropertyName(FieldsPropertyName) : FieldsPropertyName);
                 writer.WriteStartArray();
@@ -148,30 +166,30 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
                 return null;
             }
 
-            UnionCase caseInfo = null;
-            string caseName = null;
-            JArray fields = null;
+            UnionCase? caseInfo = null;
+            string? caseName = null;
+            JArray? fields = null;
 
             // start object
             reader.ReadAndAssert();
 
             while (reader.TokenType == JsonToken.PropertyName)
             {
-                string propertyName = reader.Value.ToString();
+                string propertyName = reader.Value!.ToString();
                 if (string.Equals(propertyName, CasePropertyName, StringComparison.OrdinalIgnoreCase))
                 {
                     reader.ReadAndAssert();
 
                     Union union = UnionCache.Get(objectType);
 
-                    caseName = reader.Value.ToString();
+                    caseName = reader.Value!.ToString();
 
                     caseInfo = union.Cases.SingleOrDefault(c => c.Name == caseName);
 
@@ -203,7 +221,7 @@ namespace Newtonsoft.Json.Converters
                 throw JsonSerializationException.Create(reader, "No '{0}' property with union name found.".FormatWith(CultureInfo.InvariantCulture, CasePropertyName));
             }
 
-            object[] typedFieldValues = new object[caseInfo.Fields.Length];
+            object?[] typedFieldValues = new object?[caseInfo.Fields.Length];
 
             if (caseInfo.Fields.Length > 0 && fields == null)
             {

Src/Newtonsoft.Json/Converters/EntityKeyMemberConverter.cs

@@ -50,16 +50,22 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
+            if (value == null)
+            {
+                writer.WriteNull();
+                return;
+            }
+
             EnsureReflectionObject(value.GetType());
 
-            DefaultContractResolver resolver = serializer.ContractResolver as DefaultContractResolver;
+            DefaultContractResolver? resolver = serializer.ContractResolver as DefaultContractResolver;
 
-            string keyName = (string)_reflectionObject.GetValue(value, KeyPropertyName);
-            object keyValue = _reflectionObject.GetValue(value, ValuePropertyName);
+            string keyName = (string)_reflectionObject.GetValue(value, KeyPropertyName)!;
+            object? keyValue = _reflectionObject.GetValue(value, ValuePropertyName);
 
-            Type keyValueType = keyValue?.GetType();
+            Type? keyValueType = keyValue?.GetType();
 
             writer.WriteStartObject();
             writer.WritePropertyName((resolver != null) ? resolver.GetResolvedPropertyName(KeyPropertyName) : KeyPropertyName);
@@ -71,7 +77,7 @@ namespace Newtonsoft.Json.Converters
 
             if (keyValueType != null)
             {
-                if (JsonSerializerInternalWriter.TryConvertToString(keyValue, keyValueType, out string valueJson))
+                if (JsonSerializerInternalWriter.TryConvertToString(keyValue!, keyValueType, out string? valueJson))
                 {
                     writer.WriteValue(valueJson);
                 }
@@ -92,7 +98,7 @@ namespace Newtonsoft.Json.Converters
         {
             reader.ReadAndAssert();
 
-            if (reader.TokenType != JsonToken.PropertyName || !string.Equals(reader.Value.ToString(), propertyName, StringComparison.OrdinalIgnoreCase))
+            if (reader.TokenType != JsonToken.PropertyName || !string.Equals(reader.Value?.ToString(), propertyName, StringComparison.OrdinalIgnoreCase))
             {
                 throw new JsonSerializationException("Expected JSON property '{0}'.".FormatWith(CultureInfo.InvariantCulture, propertyName));
             }
@@ -106,19 +112,19 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             EnsureReflectionObject(objectType);
 
-            object entityKeyMember = _reflectionObject.Creator();
+            object entityKeyMember = _reflectionObject.Creator!();
 
             ReadAndAssertProperty(reader, KeyPropertyName);
             reader.ReadAndAssert();
-            _reflectionObject.SetValue(entityKeyMember, KeyPropertyName, reader.Value.ToString());
+            _reflectionObject.SetValue(entityKeyMember, KeyPropertyName, reader.Value?.ToString());
 
             ReadAndAssertProperty(reader, TypePropertyName);
             reader.ReadAndAssert();
-            string type = reader.Value.ToString();
+            string? type = reader.Value?.ToString();
 
             Type t = Type.GetType(type);
 

Src/Newtonsoft.Json/Converters/ExpandoObjectConverter.cs

@@ -46,7 +46,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             // can write is set to false
         }
@@ -59,12 +59,12 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             return ReadValue(reader);
         }
 
-        private object ReadValue(JsonReader reader)
+        private object? ReadValue(JsonReader reader)
         {
             if (!reader.MoveToContent())
             {
@@ -89,7 +89,7 @@ namespace Newtonsoft.Json.Converters
 
         private object ReadList(JsonReader reader)
         {
-            IList<object> list = new List<object>();
+            IList<object?> list = new List<object?>();
 
             while (reader.Read())
             {
@@ -98,7 +98,7 @@ namespace Newtonsoft.Json.Converters
                     case JsonToken.Comment:
                         break;
                     default:
-                        object v = ReadValue(reader);
+                        object? v = ReadValue(reader);
 
                         list.Add(v);
                         break;
@@ -112,21 +112,21 @@ namespace Newtonsoft.Json.Converters
 
         private object ReadObject(JsonReader reader)
         {
-            IDictionary<string, object> expandoObject = new ExpandoObject();
+            IDictionary<string, object?> expandoObject = new ExpandoObject();
 
             while (reader.Read())
             {
                 switch (reader.TokenType)
                 {
                     case JsonToken.PropertyName:
-                        string propertyName = reader.Value.ToString();
+                        string propertyName = reader.Value!.ToString();
 
                         if (!reader.Read())
                         {
                             throw JsonSerializationException.Create(reader, "Unexpected end when reading ExpandoObject.");
                         }
 
-                        object v = ReadValue(reader);
+                        object? v = ReadValue(reader);
 
                         expandoObject[propertyName] = v;
                         break;

Src/Newtonsoft.Json/Converters/IsoDateTimeConverter.cs

@@ -37,8 +37,8 @@ namespace Newtonsoft.Json.Converters
         private const string DefaultDateTimeFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss.FFFFFFFK";
 
         private DateTimeStyles _dateTimeStyles = DateTimeStyles.RoundtripKind;
-        private string _dateTimeFormat;
-        private CultureInfo _culture;
+        private string? _dateTimeFormat;
+        private CultureInfo? _culture;
 
         /// <summary>
         /// Gets or sets the date time styles used when converting a date to and from JSON.
@@ -54,7 +54,7 @@ namespace Newtonsoft.Json.Converters
         /// Gets or sets the date time format used when converting a date to and from JSON.
         /// </summary>
         /// <value>The date time format used when converting a date to and from JSON.</value>
-        public string DateTimeFormat
+        public string? DateTimeFormat
         {
             get => _dateTimeFormat ?? string.Empty;
             set => _dateTimeFormat = (string.IsNullOrEmpty(value)) ? null : value;
@@ -76,7 +76,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             string text;
 
@@ -104,7 +104,7 @@ namespace Newtonsoft.Json.Converters
 #endif
             else
             {
-                throw new JsonSerializationException("Unexpected value when converting date. Expected DateTime or DateTimeOffset, got {0}.".FormatWith(CultureInfo.InvariantCulture, ReflectionUtils.GetObjectType(value)));
+                throw new JsonSerializationException("Unexpected value when converting date. Expected DateTime or DateTimeOffset, got {0}.".FormatWith(CultureInfo.InvariantCulture, ReflectionUtils.GetObjectType(value)!));
             }
 
             writer.WriteValue(text);
@@ -118,7 +118,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             bool nullable = ReflectionUtils.IsNullableType(objectType);
             if (reader.TokenType == JsonToken.Null)
@@ -142,7 +142,7 @@ namespace Newtonsoft.Json.Converters
 #if HAVE_DATE_TIME_OFFSET
                 if (t == typeof(DateTimeOffset))
                 {
-                    return (reader.Value is DateTimeOffset) ? reader.Value : new DateTimeOffset((DateTime)reader.Value);
+                    return (reader.Value is DateTimeOffset) ? reader.Value : new DateTimeOffset((DateTime)reader.Value!);
                 }
 
                 // converter is expected to return a DateTime
@@ -160,7 +160,7 @@ namespace Newtonsoft.Json.Converters
                 throw JsonSerializationException.Create(reader, "Unexpected token parsing date. Expected String, got {0}.".FormatWith(CultureInfo.InvariantCulture, reader.TokenType));
             }
 
-            string dateText = reader.Value.ToString();
+            string? dateText = reader.Value?.ToString();
 
             if (string.IsNullOrEmpty(dateText) && nullable)
             {

Src/Newtonsoft.Json/Converters/JavaScriptDateTimeConverter.cs

@@ -40,7 +40,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             long ticks;
 
@@ -74,7 +74,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing property value of the JSON that is being converted.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -86,12 +86,12 @@ namespace Newtonsoft.Json.Converters
                 return null;
             }
 
-            if (reader.TokenType != JsonToken.StartConstructor || !string.Equals(reader.Value.ToString(), "Date", StringComparison.Ordinal))
+            if (reader.TokenType != JsonToken.StartConstructor || !string.Equals(reader.Value?.ToString(), "Date", StringComparison.Ordinal))
             {
                 throw JsonSerializationException.Create(reader, "Unexpected token or value when parsing date. Token: {0}, Value: {1}".FormatWith(CultureInfo.InvariantCulture, reader.TokenType, reader.Value));
             }
 
-            if (!JavaScriptUtils.TryGetDateFromConstructorJson(reader, out DateTime d, out string errorMessage))
+            if (!JavaScriptUtils.TryGetDateFromConstructorJson(reader, out DateTime d, out string? errorMessage))
             {
                 throw JsonSerializationException.Create(reader, errorMessage);
             }

Src/Newtonsoft.Json/Converters/KeyValuePairConverter.cs

@@ -56,11 +56,17 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
+            if (value == null)
+            {
+                writer.WriteNull();
+                return;
+            }
+
             ReflectionObject reflectionObject = ReflectionObjectPerType.Get(value.GetType());
 
-            DefaultContractResolver resolver = serializer.ContractResolver as DefaultContractResolver;
+            DefaultContractResolver? resolver = serializer.ContractResolver as DefaultContractResolver;
 
             writer.WriteStartObject();
             writer.WritePropertyName((resolver != null) ? resolver.GetResolvedPropertyName(KeyName) : KeyName);
@@ -78,7 +84,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -90,8 +96,8 @@ namespace Newtonsoft.Json.Converters
                 return null;
             }
 
-            object key = null;
-            object value = null;
+            object? key = null;
+            object? value = null;
 
             reader.ReadAndAssert();
 
@@ -105,7 +111,7 @@ namespace Newtonsoft.Json.Converters
 
             while (reader.TokenType == JsonToken.PropertyName)
             {
-                string propertyName = reader.Value.ToString();
+                string propertyName = reader.Value!.ToString();
                 if (string.Equals(propertyName, KeyName, StringComparison.OrdinalIgnoreCase))
                 {
                     reader.ReadForTypeAndAssert(keyContract, false);
@@ -126,7 +132,7 @@ namespace Newtonsoft.Json.Converters
                 reader.ReadAndAssert();
             }
 
-            return reflectionObject.Creator(key, value);
+            return reflectionObject.Creator!(key, value);
         }
 
         /// <summary>

Src/Newtonsoft.Json/Converters/RegexConverter.cs

@@ -47,7 +47,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -84,7 +84,7 @@ namespace Newtonsoft.Json.Converters
             // 'l' to make \w, \W, etc. locale dependent, 's' for dotall mode 
             // ('.' matches everything), and 'u' to make \w, \W, etc. match unicode.
 
-            string options = null;
+            string? options = null;
 
             if (HasFlag(regex.Options, RegexOptions.IgnoreCase))
             {
@@ -114,7 +114,7 @@ namespace Newtonsoft.Json.Converters
 
         private void WriteJson(JsonWriter writer, Regex regex, JsonSerializer serializer)
         {
-            DefaultContractResolver resolver = serializer.ContractResolver as DefaultContractResolver;
+            DefaultContractResolver? resolver = serializer.ContractResolver as DefaultContractResolver;
 
             writer.WriteStartObject();
             writer.WritePropertyName((resolver != null) ? resolver.GetResolvedPropertyName(PatternName) : PatternName);
@@ -132,7 +132,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             switch (reader.TokenType)
             {
@@ -149,7 +149,7 @@ namespace Newtonsoft.Json.Converters
 
         private object ReadRegexString(JsonReader reader)
         {
-            string regexText = (string)reader.Value;
+            string regexText = (string)reader.Value!;
 
             if (regexText.Length > 0 && regexText[0] == '/')
             {
@@ -171,7 +171,7 @@ namespace Newtonsoft.Json.Converters
 
         private Regex ReadRegexObject(JsonReader reader, JsonSerializer serializer)
         {
-            string pattern = null;
+            string? pattern = null;
             RegexOptions? options = null;
 
             while (reader.Read())
@@ -179,7 +179,7 @@ namespace Newtonsoft.Json.Converters
                 switch (reader.TokenType)
                 {
                     case JsonToken.PropertyName:
-                        string propertyName = reader.Value.ToString();
+                        string propertyName = reader.Value!.ToString();
 
                         if (!reader.Read())
                         {
@@ -188,7 +188,7 @@ namespace Newtonsoft.Json.Converters
 
                         if (string.Equals(propertyName, PatternName, StringComparison.OrdinalIgnoreCase))
                         {
-                            pattern = (string)reader.Value;
+                            pattern = (string?)reader.Value;
                         }
                         else if (string.Equals(propertyName, OptionsName, StringComparison.OrdinalIgnoreCase))
                         {

Src/Newtonsoft.Json/Converters/StringEnumConverter.cs

@@ -82,7 +82,7 @@ namespace Newtonsoft.Json.Converters
         /// Gets or sets the naming strategy used to resolve how enum text is written.
         /// </summary>
         /// <value>The naming strategy used to resolve how enum text is written.</value>
-        public NamingStrategy NamingStrategy { get; set; }
+        public NamingStrategy? NamingStrategy { get; set; }
 
         /// <summary>
         /// Gets or sets a value indicating whether integer values are allowed when serializing and deserializing.
@@ -175,7 +175,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -185,7 +185,7 @@ namespace Newtonsoft.Json.Converters
 
             Enum e = (Enum)value;
 
-            if (!EnumUtils.TryToString(e.GetType(), value, NamingStrategy, out string enumName))
+            if (!EnumUtils.TryToString(e.GetType(), value, NamingStrategy, out string? enumName))
             {
                 if (!AllowIntegerValues)
                 {
@@ -209,7 +209,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -228,14 +228,14 @@ namespace Newtonsoft.Json.Converters
             {
                 if (reader.TokenType == JsonToken.String)
                 {
-                    string enumText = reader.Value.ToString();
+                    string? enumText = reader.Value?.ToString();
 
-                    if (enumText == string.Empty && isNullable)
+                    if (string.IsNullOrEmpty(enumText) && isNullable)
                     {
                         return null;
                     }
 
-                    return EnumUtils.ParseEnum(t, NamingStrategy, enumText, !AllowIntegerValues);
+                    return EnumUtils.ParseEnum(t, NamingStrategy, enumText!, !AllowIntegerValues);
                 }
 
                 if (reader.TokenType == JsonToken.Integer)

Src/Newtonsoft.Json/Converters/UnixDateTimeConverter.cs

@@ -42,7 +42,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             long seconds;
 
@@ -77,7 +77,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing property value of the JSON that is being converted.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             bool nullable = ReflectionUtils.IsNullable(objectType);
             if (reader.TokenType == JsonToken.Null)
@@ -94,11 +94,11 @@ namespace Newtonsoft.Json.Converters
 
             if (reader.TokenType == JsonToken.Integer)
             {
-                seconds = (long)reader.Value;
+                seconds = (long)reader.Value!;
             }
             else if (reader.TokenType == JsonToken.String)
             {
-                if (!long.TryParse((string)reader.Value, out seconds))
+                if (!long.TryParse((string)reader.Value!, out seconds))
                 {
                     throw JsonSerializationException.Create(reader, "Cannot convert invalid value to {0}.".FormatWith(CultureInfo.InvariantCulture, objectType));
                 }

Src/Newtonsoft.Json/Converters/VersionConverter.cs

@@ -40,7 +40,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -64,7 +64,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing property value of the JSON that is being converted.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             if (reader.TokenType == JsonToken.Null)
             {
@@ -76,7 +76,7 @@ namespace Newtonsoft.Json.Converters
                 {
                     try
                     {
-                        Version v = new Version((string)reader.Value);
+                        Version v = new Version((string)reader.Value!);
                         return v;
                     }
                     catch (Exception ex)

Src/Newtonsoft.Json/Converters/XmlNodeConverter.cs

@@ -38,6 +38,7 @@ using System.Xml.Linq;
 #endif
 using Newtonsoft.Json.Utilities;
 using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Newtonsoft.Json.Converters
 {
@@ -53,44 +54,44 @@ namespace Newtonsoft.Json.Converters
             _document = document;
         }
 
-        public IXmlNode CreateComment(string data)
+        public IXmlNode CreateComment(string? data)
         {
             return new XmlNodeWrapper(_document.CreateComment(data));
         }
 
-        public IXmlNode CreateTextNode(string text)
+        public IXmlNode CreateTextNode(string? text)
         {
             return new XmlNodeWrapper(_document.CreateTextNode(text));
         }
 
-        public IXmlNode CreateCDataSection(string data)
+        public IXmlNode CreateCDataSection(string? data)
         {
             return new XmlNodeWrapper(_document.CreateCDataSection(data));
         }
 
-        public IXmlNode CreateWhitespace(string text)
+        public IXmlNode CreateWhitespace(string? text)
         {
             return new XmlNodeWrapper(_document.CreateWhitespace(text));
         }
 
-        public IXmlNode CreateSignificantWhitespace(string text)
+        public IXmlNode CreateSignificantWhitespace(string? text)
         {
             return new XmlNodeWrapper(_document.CreateSignificantWhitespace(text));
         }
 
-        public IXmlNode CreateXmlDeclaration(string version, string encoding, string standalone)
+        public IXmlNode CreateXmlDeclaration(string? version, string? encoding, string? standalone)
         {
             return new XmlDeclarationWrapper(_document.CreateXmlDeclaration(version, encoding, standalone));
         }
 
 #if HAVE_XML_DOCUMENT_TYPE
-        public IXmlNode CreateXmlDocumentType(string name, string publicId, string systemId, string internalSubset)
+        public IXmlNode CreateXmlDocumentType(string? name, string? publicId, string? systemId, string? internalSubset)
         {
             return new XmlDocumentTypeWrapper(_document.CreateDocumentType(name, publicId, systemId, null));
         }
 #endif
 
-        public IXmlNode CreateProcessingInstruction(string target, string data)
+        public IXmlNode CreateProcessingInstruction(string target, string? data)
         {
             return new XmlNodeWrapper(_document.CreateProcessingInstruction(target, data));
         }
@@ -105,7 +106,7 @@ namespace Newtonsoft.Json.Converters
             return new XmlElementWrapper(_document.CreateElement(qualifiedName, namespaceUri));
         }
 
-        public IXmlNode CreateAttribute(string name, string value)
+        public IXmlNode CreateAttribute(string name, string? value)
         {
             XmlNodeWrapper attribute = new XmlNodeWrapper(_document.CreateAttribute(name));
             attribute.Value = value;
@@ -113,7 +114,7 @@ namespace Newtonsoft.Json.Converters
             return attribute;
         }
 
-        public IXmlNode CreateAttribute(string qualifiedName, string namespaceUri, string value)
+        public IXmlNode CreateAttribute(string qualifiedName, string namespaceUri, string? value)
         {
             XmlNodeWrapper attribute = new XmlNodeWrapper(_document.CreateAttribute(qualifiedName, namespaceUri));
             attribute.Value = value;
@@ -121,7 +122,7 @@ namespace Newtonsoft.Json.Converters
             return attribute;
         }
 
-        public IXmlElement DocumentElement
+        public IXmlElement? DocumentElement
         {
             get
             {
@@ -149,7 +150,7 @@ namespace Newtonsoft.Json.Converters
         {
             XmlNodeWrapper xmlAttributeWrapper = (XmlNodeWrapper)attribute;
 
-            _element.SetAttributeNode((XmlAttribute)xmlAttributeWrapper.WrappedNode);
+            _element.SetAttributeNode((XmlAttribute)xmlAttributeWrapper.WrappedNode!);
         }
 
         public string GetPrefixOfNamespace(string namespaceUri)
@@ -204,26 +205,26 @@ namespace Newtonsoft.Json.Converters
 
         public string InternalSubset => _documentType.InternalSubset;
 
-        public override string LocalName => "DOCTYPE";
+        public override string? LocalName => "DOCTYPE";
     }
 #endif
 
     internal class XmlNodeWrapper : IXmlNode
     {
         private readonly XmlNode _node;
-        private List<IXmlNode> _childNodes;
-        private List<IXmlNode> _attributes;
+        private List<IXmlNode>? _childNodes;
+        private List<IXmlNode>? _attributes;
 
         public XmlNodeWrapper(XmlNode node)
         {
             _node = node;
         }
 
-        public object WrappedNode => _node;
+        public object? WrappedNode => _node;
 
         public XmlNodeType NodeType => _node.NodeType;
 
-        public virtual string LocalName => _node.LocalName;
+        public virtual string? LocalName => _node.LocalName;
 
         public List<IXmlNode> ChildNodes
         {
@@ -309,7 +310,7 @@ namespace Newtonsoft.Json.Converters
             }
         }
 
-        public IXmlNode ParentNode
+        public IXmlNode? ParentNode
         {
             get
             {
@@ -324,7 +325,7 @@ namespace Newtonsoft.Json.Converters
             }
         }
 
-        public string Value
+        public string? Value
         {
             get => _node.Value;
             set => _node.Value = value;
@@ -340,7 +341,7 @@ namespace Newtonsoft.Json.Converters
             return newChild;
         }
 
-        public string NamespaceUri => _node.NamespaceURI;
+        public string? NamespaceUri => _node.NamespaceURI;
     }
 #endif
 #endregion
@@ -348,22 +349,22 @@ namespace Newtonsoft.Json.Converters
 #region Interfaces
     internal interface IXmlDocument : IXmlNode
     {
-        IXmlNode CreateComment(string text);
-        IXmlNode CreateTextNode(string text);
-        IXmlNode CreateCDataSection(string data);
-        IXmlNode CreateWhitespace(string text);
-        IXmlNode CreateSignificantWhitespace(string text);
-        IXmlNode CreateXmlDeclaration(string version, string encoding, string standalone);
+        IXmlNode CreateComment(string? text);
+        IXmlNode CreateTextNode(string? text);
+        IXmlNode CreateCDataSection(string? data);
+        IXmlNode CreateWhitespace(string? text);
+        IXmlNode CreateSignificantWhitespace(string? text);
+        IXmlNode CreateXmlDeclaration(string? version, string? encoding, string? standalone);
 #if HAVE_XML_DOCUMENT_TYPE
-        IXmlNode CreateXmlDocumentType(string name, string publicId, string systemId, string internalSubset);
+        IXmlNode CreateXmlDocumentType(string? name, string? publicId, string? systemId, string? internalSubset);
 #endif
-        IXmlNode CreateProcessingInstruction(string target, string data);
+        IXmlNode CreateProcessingInstruction(string target, string? data);
         IXmlElement CreateElement(string elementName);
         IXmlElement CreateElement(string qualifiedName, string namespaceUri);
-        IXmlNode CreateAttribute(string name, string value);
-        IXmlNode CreateAttribute(string qualifiedName, string namespaceUri, string value);
+        IXmlNode CreateAttribute(string name, string? value);
+        IXmlNode CreateAttribute(string qualifiedName, string namespaceUri, string? value);
 
-        IXmlElement DocumentElement { get; }
+        IXmlElement? DocumentElement { get; }
     }
 
     internal interface IXmlDeclaration : IXmlNode
@@ -391,14 +392,14 @@ namespace Newtonsoft.Json.Converters
     internal interface IXmlNode
     {
         XmlNodeType NodeType { get; }
-        string LocalName { get; }
+        string? LocalName { get; }
         List<IXmlNode> ChildNodes { get; }
         List<IXmlNode> Attributes { get; }
-        IXmlNode ParentNode { get; }
-        string Value { get; set; }
+        IXmlNode? ParentNode { get; }
+        string? Value { get; set; }
         IXmlNode AppendChild(IXmlNode newChild);
-        string NamespaceUri { get; }
-        object WrappedNode { get; }
+        string? NamespaceUri { get; }
+        object? WrappedNode { get; }
     }
 #endregion
 
@@ -449,12 +450,12 @@ namespace Newtonsoft.Json.Converters
 
         public string InternalSubset => _documentType.InternalSubset;
 
-        public override string LocalName => "DOCTYPE";
+        public override string? LocalName => "DOCTYPE";
     }
 
     internal class XDocumentWrapper : XContainerWrapper, IXmlDocument
     {
-        private XDocument Document => (XDocument)WrappedNode;
+        private XDocument Document => (XDocument)WrappedNode!;
 
         public XDocumentWrapper(XDocument document)
             : base(document)
@@ -488,42 +489,42 @@ namespace Newtonsoft.Json.Converters
             }
         }
 
-        public IXmlNode CreateComment(string text)
+        public IXmlNode CreateComment(string? text)
         {
             return new XObjectWrapper(new XComment(text));
         }
 
-        public IXmlNode CreateTextNode(string text)
+        public IXmlNode CreateTextNode(string? text)
         {
             return new XObjectWrapper(new XText(text));
         }
 
-        public IXmlNode CreateCDataSection(string data)
+        public IXmlNode CreateCDataSection(string? data)
         {
             return new XObjectWrapper(new XCData(data));
         }
 
-        public IXmlNode CreateWhitespace(string text)
+        public IXmlNode CreateWhitespace(string? text)
         {
             return new XObjectWrapper(new XText(text));
         }
 
-        public IXmlNode CreateSignificantWhitespace(string text)
+        public IXmlNode CreateSignificantWhitespace(string? text)
         {
             return new XObjectWrapper(new XText(text));
         }
 
-        public IXmlNode CreateXmlDeclaration(string version, string encoding, string standalone)
+        public IXmlNode CreateXmlDeclaration(string? version, string? encoding, string? standalone)
         {
             return new XDeclarationWrapper(new XDeclaration(version, encoding, standalone));
         }
 
-        public IXmlNode CreateXmlDocumentType(string name, string publicId, string systemId, string internalSubset)
+        public IXmlNode CreateXmlDocumentType(string? name, string? publicId, string? systemId, string? internalSubset)
         {
             return new XDocumentTypeWrapper(new XDocumentType(name, publicId, systemId, internalSubset));
         }
 
-        public IXmlNode CreateProcessingInstruction(string target, string data)
+        public IXmlNode CreateProcessingInstruction(string target, string? data)
         {
             return new XProcessingInstructionWrapper(new XProcessingInstruction(target, data));
         }
@@ -539,18 +540,18 @@ namespace Newtonsoft.Json.Converters
             return new XElementWrapper(new XElement(XName.Get(localName, namespaceUri)));
         }
 
-        public IXmlNode CreateAttribute(string name, string value)
+        public IXmlNode CreateAttribute(string name, string? value)
         {
             return new XAttributeWrapper(new XAttribute(name, value));
         }
 
-        public IXmlNode CreateAttribute(string qualifiedName, string namespaceUri, string value)
+        public IXmlNode CreateAttribute(string qualifiedName, string namespaceUri, string? value)
         {
             string localName = MiscellaneousUtils.GetLocalName(qualifiedName);
             return new XAttributeWrapper(new XAttribute(XName.Get(localName, namespaceUri), value));
         }
 
-        public IXmlElement DocumentElement
+        public IXmlElement? DocumentElement
         {
             get
             {
@@ -579,20 +580,20 @@ namespace Newtonsoft.Json.Converters
 
     internal class XTextWrapper : XObjectWrapper
     {
-        private XText Text => (XText)WrappedNode;
+        private XText Text => (XText)WrappedNode!;
 
         public XTextWrapper(XText text)
             : base(text)
         {
         }
 
-        public override string Value
+        public override string? Value
         {
             get => Text.Value;
             set => Text.Value = value;
         }
 
-        public override IXmlNode ParentNode
+        public override IXmlNode? ParentNode
         {
             get
             {
@@ -608,20 +609,20 @@ namespace Newtonsoft.Json.Converters
 
     internal class XCommentWrapper : XObjectWrapper
     {
-        private XComment Text => (XComment)WrappedNode;
+        private XComment Text => (XComment)WrappedNode!;
 
         public XCommentWrapper(XComment text)
             : base(text)
         {
         }
 
-        public override string Value
+        public override string? Value
         {
             get => Text.Value;
             set => Text.Value = value;
         }
 
-        public override IXmlNode ParentNode
+        public override IXmlNode? ParentNode
         {
             get
             {
@@ -637,16 +638,16 @@ namespace Newtonsoft.Json.Converters
 
     internal class XProcessingInstructionWrapper : XObjectWrapper
     {
-        private XProcessingInstruction ProcessingInstruction => (XProcessingInstruction)WrappedNode;
+        private XProcessingInstruction ProcessingInstruction => (XProcessingInstruction)WrappedNode!;
 
         public XProcessingInstructionWrapper(XProcessingInstruction processingInstruction)
             : base(processingInstruction)
         {
         }
 
-        public override string LocalName => ProcessingInstruction.Target;
+        public override string? LocalName => ProcessingInstruction.Target;
 
-        public override string Value
+        public override string? Value
         {
             get => ProcessingInstruction.Data;
             set => ProcessingInstruction.Data = value;
@@ -655,9 +656,9 @@ namespace Newtonsoft.Json.Converters
 
     internal class XContainerWrapper : XObjectWrapper
     {
-        private List<IXmlNode> _childNodes;
+        private List<IXmlNode>? _childNodes;
 
-        private XContainer Container => (XContainer)WrappedNode;
+        private XContainer Container => (XContainer)WrappedNode!;
 
         public XContainerWrapper(XContainer container)
             : base(container)
@@ -692,7 +693,7 @@ namespace Newtonsoft.Json.Converters
 
         protected virtual bool HasChildNodes => Container.LastNode != null;
 
-        public override IXmlNode ParentNode
+        public override IXmlNode? ParentNode
         {
             get
             {
@@ -761,26 +762,26 @@ namespace Newtonsoft.Json.Converters
 
     internal class XObjectWrapper : IXmlNode
     {
-        private readonly XObject _xmlObject;
+        private readonly XObject? _xmlObject;
 
-        public XObjectWrapper(XObject xmlObject)
+        public XObjectWrapper(XObject? xmlObject)
         {
             _xmlObject = xmlObject;
         }
 
-        public object WrappedNode => _xmlObject;
+        public object? WrappedNode => _xmlObject;
 
-        public virtual XmlNodeType NodeType => _xmlObject.NodeType;
+        public virtual XmlNodeType NodeType => _xmlObject?.NodeType ?? XmlNodeType.None;
 
-        public virtual string LocalName => null;
+        public virtual string? LocalName => null;
 
         public virtual List<IXmlNode> ChildNodes => XmlNodeConverter.EmptyChildNodes;
 
         public virtual List<IXmlNode> Attributes => XmlNodeConverter.EmptyChildNodes;
 
-        public virtual IXmlNode ParentNode => null;
+        public virtual IXmlNode? ParentNode => null;
 
-        public virtual string Value
+        public virtual string? Value
         {
             get => null;
             set => throw new InvalidOperationException();
@@ -791,29 +792,29 @@ namespace Newtonsoft.Json.Converters
             throw new InvalidOperationException();
         }
 
-        public virtual string NamespaceUri => null;
+        public virtual string? NamespaceUri => null;
     }
 
     internal class XAttributeWrapper : XObjectWrapper
     {
-        private XAttribute Attribute => (XAttribute)WrappedNode;
+        private XAttribute Attribute => (XAttribute)WrappedNode!;
 
         public XAttributeWrapper(XAttribute attribute)
             : base(attribute)
         {
         }
 
-        public override string Value
+        public override string? Value
         {
             get => Attribute.Value;
             set => Attribute.Value = value;
         }
 
-        public override string LocalName => Attribute.Name.LocalName;
+        public override string? LocalName => Attribute.Name.LocalName;
 
-        public override string NamespaceUri => Attribute.Name.NamespaceName;
+        public override string? NamespaceUri => Attribute.Name.NamespaceName;
 
-        public override IXmlNode ParentNode
+        public override IXmlNode? ParentNode
         {
             get
             {
@@ -829,9 +830,9 @@ namespace Newtonsoft.Json.Converters
 
     internal class XElementWrapper : XContainerWrapper, IXmlElement
     {
-        private List<IXmlNode> _attributes;
+        private List<IXmlNode>? _attributes;
 
-        private XElement Element => (XElement)WrappedNode;
+        private XElement Element => (XElement)WrappedNode!;
 
         public XElementWrapper(XElement element)
             : base(element)
@@ -853,7 +854,7 @@ namespace Newtonsoft.Json.Converters
                 // cache results to prevent multiple reads which kills perf in large documents
                 if (_attributes == null)
                 {
-                    if (!Element.HasAttributes && !HasImplicitNamespaceAttribute(NamespaceUri))
+                    if (!Element.HasAttributes && !HasImplicitNamespaceAttribute(NamespaceUri!))
                     {
                         _attributes = XmlNodeConverter.EmptyChildNodes;
                     }
@@ -867,7 +868,7 @@ namespace Newtonsoft.Json.Converters
 
                         // ensure elements created with a namespace but no namespace attribute are converted correctly
                         // e.g. new XElement("{http://example.com}MyElement");
-                        string namespaceUri = NamespaceUri;
+                        string namespaceUri = NamespaceUri!;
                         if (HasImplicitNamespaceAttribute(namespaceUri))
                         {
                             _attributes.Insert(0, new XAttributeWrapper(new XAttribute("xmlns", namespaceUri)));
@@ -915,15 +916,15 @@ namespace Newtonsoft.Json.Converters
             return result;
         }
 
-        public override string Value
+        public override string? Value
         {
             get => Element.Value;
             set => Element.Value = value;
         }
 
-        public override string LocalName => Element.Name.LocalName;
+        public override string? LocalName => Element.Name.LocalName;
 
-        public override string NamespaceUri => Element.Name.NamespaceName;
+        public override string? NamespaceUri => Element.Name.NamespaceName;
 
         public string GetPrefixOfNamespace(string namespaceUri)
         {
@@ -954,7 +955,7 @@ namespace Newtonsoft.Json.Converters
         /// Gets or sets the name of the root element to insert when deserializing to XML if the JSON structure has produced multiple root elements.
         /// </summary>
         /// <value>The name of the deserialized root element.</value>
-        public string DeserializeRootElementName { get; set; }
+        public string? DeserializeRootElementName { get; set; }
 
         /// <summary>
         /// Gets or sets a value to indicate whether to write the Json.NET array attribute.
@@ -985,7 +986,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <param name="value">The value.</param>
-        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (value == null)
             {
@@ -1031,9 +1032,9 @@ namespace Newtonsoft.Json.Converters
 
         private void PushParentNamespaces(IXmlNode node, XmlNamespaceManager manager)
         {
-            List<IXmlNode> parentElements = null;
+            List<IXmlNode>? parentElements = null;
 
-            IXmlNode parent = node;
+            IXmlNode? parent = node;
             while ((parent = parent.ParentNode) != null)
             {
                 if (parent.NodeType == XmlNodeType.Element)
@@ -1067,7 +1068,7 @@ namespace Newtonsoft.Json.Converters
 
         private string ResolveFullName(IXmlNode node, XmlNamespaceManager manager)
         {
-            string prefix = (node.NamespaceUri == null || (node.LocalName == "xmlns" && node.NamespaceUri == "http://www.w3.org/2000/xmlns/"))
+            string? prefix = (node.NamespaceUri == null || (node.LocalName == "xmlns" && node.NamespaceUri == "http://www.w3.org/2000/xmlns/"))
                 ? null
                 : manager.LookupPrefix(node.NamespaceUri);
 
@@ -1160,9 +1161,9 @@ namespace Newtonsoft.Json.Converters
 
                     // value of dictionary will be a single IXmlNode when there is one for a name,
                     // or a List<IXmlNode> when there are multiple
-                    Dictionary<string, object> nodesGroupedByName = null;
+                    Dictionary<string, object>? nodesGroupedByName = null;
 
-                    string nodeName = null;
+                    string? nodeName = null;
 
                     for (int i = 0; i < node.ChildNodes.Count; i++)
                     {
@@ -1208,7 +1209,7 @@ namespace Newtonsoft.Json.Converters
                             {
                                 if (!(value is List<IXmlNode> nodes))
                                 {
-                                    nodes = new List<IXmlNode> {(IXmlNode)value};
+                                    nodes = new List<IXmlNode> {(IXmlNode)value!};
                                     nodesGroupedByName[currentNodeName] = nodes;
                                 }
 
@@ -1219,7 +1220,7 @@ namespace Newtonsoft.Json.Converters
 
                     if (nodesGroupedByName == null)
                     {
-                        WriteGroupedNodes(writer, manager, writePropertyName, node.ChildNodes, nodeName);
+                        WriteGroupedNodes(writer, manager, writePropertyName, node.ChildNodes, nodeName!);
                     }
                     else
                     {
@@ -1315,7 +1316,11 @@ namespace Newtonsoft.Json.Converters
                                 string namespacePrefix = (attribute.LocalName != "xmlns")
                                     ? XmlConvert.DecodeName(attribute.LocalName)
                                     : string.Empty;
-                                string namespaceUri = attribute.Value;
+                                string? namespaceUri = attribute.Value;
+                                if (namespaceUri == null)
+                                {
+                                    throw new JsonSerializationException("Namespace attribute must have a value.");
+                                }
 
                                 manager.AddNamespace(namespacePrefix, namespaceUri);
                             }
@@ -1473,7 +1478,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             switch (reader.TokenType)
             {
@@ -1486,8 +1491,8 @@ namespace Newtonsoft.Json.Converters
             }
 
             XmlNamespaceManager manager = new XmlNamespaceManager(new NameTable());
-            IXmlDocument document = null;
-            IXmlNode rootNode = null;
+            IXmlDocument? document = null;
+            IXmlNode? rootNode = null;
 
 #if HAVE_XLINQ
             if (typeof(XObject).IsAssignableFrom(objectType))
@@ -1545,7 +1550,7 @@ namespace Newtonsoft.Json.Converters
 #if HAVE_XLINQ
             if (objectType == typeof(XElement))
             {
-                XElement element = (XElement)document.DocumentElement.WrappedNode;
+                XElement element = (XElement)document.DocumentElement!.WrappedNode!;
                 element.Remove();
 
                 return element;
@@ -1554,7 +1559,7 @@ namespace Newtonsoft.Json.Converters
 #if HAVE_XML_DOCUMENT
             if (objectType == typeof(XmlElement))
             {
-                return document.DocumentElement.WrappedNode;
+                return document.DocumentElement!.WrappedNode;
             }
 #endif
 
@@ -1616,8 +1621,8 @@ namespace Newtonsoft.Json.Converters
                 throw JsonSerializationException.Create(reader, "XmlNodeConverter cannot convert JSON with an empty property name to XML.");
             }
 
-            Dictionary<string, string> attributeNameValues = null;
-            string elementPrefix = null;
+            Dictionary<string, string?>? attributeNameValues = null;
+            string? elementPrefix = null;
 
             if (!EncodeSpecialCharacters)
             {
@@ -1629,7 +1634,7 @@ namespace Newtonsoft.Json.Converters
                 if (propertyName.StartsWith('@'))
                 {
                     string attributeName = propertyName.Substring(1);
-                    string attributePrefix = MiscellaneousUtils.GetPrefix(attributeName);
+                    string? attributePrefix = MiscellaneousUtils.GetPrefix(attributeName);
 
                     AddAttribute(reader, document, currentNode, propertyName, attributeName, manager, attributePrefix);
                     return;
@@ -1666,7 +1671,7 @@ namespace Newtonsoft.Json.Converters
             CreateElement(reader, document, currentNode, propertyName, manager, elementPrefix, attributeNameValues);
         }
 
-        private void CreateElement(JsonReader reader, IXmlDocument document, IXmlNode currentNode, string elementName, XmlNamespaceManager manager, string elementPrefix, Dictionary<string, string> attributeNameValues)
+        private void CreateElement(JsonReader reader, IXmlDocument document, IXmlNode currentNode, string elementName, XmlNamespaceManager manager, string? elementPrefix, Dictionary<string, string?>? attributeNameValues)
         {
             IXmlElement element = CreateElement(elementName, document, elementPrefix, manager);
 
@@ -1675,10 +1680,10 @@ namespace Newtonsoft.Json.Converters
             if (attributeNameValues != null)
             {
                 // add attributes to newly created element
-                foreach (KeyValuePair<string, string> nameValue in attributeNameValues)
+                foreach (KeyValuePair<string, string?> nameValue in attributeNameValues)
                 {
                     string encodedName = XmlConvert.EncodeName(nameValue.Key);
-                    string attributePrefix = MiscellaneousUtils.GetPrefix(nameValue.Key);
+                    string? attributePrefix = MiscellaneousUtils.GetPrefix(nameValue.Key);
 
                     IXmlNode attribute = (!string.IsNullOrEmpty(attributePrefix)) ? document.CreateAttribute(encodedName, manager.LookupNamespace(attributePrefix) ?? string.Empty, nameValue.Value) : document.CreateAttribute(encodedName, nameValue.Value);
 
@@ -1694,7 +1699,7 @@ namespace Newtonsoft.Json.Converters
                 case JsonToken.Boolean:
                 case JsonToken.Date:
                 case JsonToken.Bytes:
-                    string text = ConvertTokenToXmlValue(reader);
+                    string? text = ConvertTokenToXmlValue(reader);
                     if (text != null)
                     {
                         element.AppendChild(document.CreateTextNode(text));
@@ -1718,7 +1723,7 @@ namespace Newtonsoft.Json.Converters
             }
         }
 
-        private static void AddAttribute(JsonReader reader, IXmlDocument document, IXmlNode currentNode, string propertyName, string attributeName, XmlNamespaceManager manager, string attributePrefix)
+        private static void AddAttribute(JsonReader reader, IXmlDocument document, IXmlNode currentNode, string propertyName, string attributeName, XmlNamespaceManager manager, string? attributePrefix)
         {
             if (currentNode.NodeType == XmlNodeType.Document)
             {
@@ -1726,7 +1731,7 @@ namespace Newtonsoft.Json.Converters
             }
 
             string encodedName = XmlConvert.EncodeName(attributeName);
-            string attributeValue = ConvertTokenToXmlValue(reader);
+            string? attributeValue = ConvertTokenToXmlValue(reader);
 
             IXmlNode attribute = (!string.IsNullOrEmpty(attributePrefix))
                 ? document.CreateAttribute(encodedName, manager.LookupNamespace(attributePrefix), attributeValue)
@@ -1735,7 +1740,7 @@ namespace Newtonsoft.Json.Converters
             ((IXmlElement)currentNode).SetAttributeNode(attribute);
         }
 
-        private static string ConvertTokenToXmlValue(JsonReader reader)
+        private static string? ConvertTokenToXmlValue(JsonReader reader)
         {
             switch (reader.TokenType)
             {
@@ -1782,7 +1787,7 @@ namespace Newtonsoft.Json.Converters
 #endif
                 }
                 case JsonToken.Bytes:
-                    return Convert.ToBase64String((byte[])reader.Value);
+                    return Convert.ToBase64String((byte[])reader.Value!);
                 case JsonToken.Null:
                     return null;
                 default:
@@ -1792,7 +1797,7 @@ namespace Newtonsoft.Json.Converters
 
         private void ReadArrayElements(JsonReader reader, IXmlDocument document, string propertyName, IXmlNode currentNode, XmlNamespaceManager manager)
         {
-            string elementPrefix = MiscellaneousUtils.GetPrefix(propertyName);
+            string? elementPrefix = MiscellaneousUtils.GetPrefix(propertyName);
 
             IXmlElement nestedArrayElement = CreateElement(propertyName, document, elementPrefix, manager);
 
@@ -1858,9 +1863,9 @@ namespace Newtonsoft.Json.Converters
             return true;
         }
 
-        private Dictionary<string, string> ReadAttributeElements(JsonReader reader, XmlNamespaceManager manager)
+        private Dictionary<string, string?>? ReadAttributeElements(JsonReader reader, XmlNamespaceManager manager)
         {
-            Dictionary<string, string> attributeNameValues = null;
+            Dictionary<string, string?>? attributeNameValues = null;
             bool finished = false;
 
             // read properties until first non-attribute is encountered
@@ -1869,19 +1874,19 @@ namespace Newtonsoft.Json.Converters
                 switch (reader.TokenType)
                 {
                     case JsonToken.PropertyName:
-                        string attributeName = reader.Value.ToString();
+                        string attributeName = reader.Value!.ToString();
 
                         if (!string.IsNullOrEmpty(attributeName))
                         {
                             char firstChar = attributeName[0];
-                            string attributeValue;
+                            string? attributeValue;
 
                             switch (firstChar)
                             {
                                 case '@':
                                     if (attributeNameValues == null)
                                     {
-                                        attributeNameValues = new Dictionary<string, string>();
+                                        attributeNameValues = new Dictionary<string, string?>();
                                     }
 
                                     attributeName = attributeName.Substring(1);
@@ -1889,7 +1894,7 @@ namespace Newtonsoft.Json.Converters
                                     attributeValue = ConvertTokenToXmlValue(reader);
                                     attributeNameValues.Add(attributeName, attributeValue);
 
-                                    if (IsNamespaceAttribute(attributeName, out string namespacePrefix))
+                                    if (IsNamespaceAttribute(attributeName, out string? namespacePrefix))
                                     {
                                         manager.AddNamespace(namespacePrefix, attributeValue);
                                     }
@@ -1909,7 +1914,7 @@ namespace Newtonsoft.Json.Converters
                                             {
                                                 if (attributeNameValues == null)
                                                 {
-                                                    attributeNameValues = new Dictionary<string, string>();
+                                                    attributeNameValues = new Dictionary<string, string?>();
                                                 }
 
                                                 // ensure that the prefix used is free
@@ -1941,7 +1946,7 @@ namespace Newtonsoft.Json.Converters
 
                                             if (attributeNameValues == null)
                                             {
-                                                attributeNameValues = new Dictionary<string, string>();
+                                                attributeNameValues = new Dictionary<string, string?>();
                                             }
 
                                             attributeValue = reader.Value?.ToString();
@@ -1979,12 +1984,12 @@ namespace Newtonsoft.Json.Converters
         {
             if (propertyName == DeclarationName)
             {
-                string version = null;
-                string encoding = null;
-                string standalone = null;
+                string? version = null;
+                string? encoding = null;
+                string? standalone = null;
                 while (reader.Read() && reader.TokenType != JsonToken.EndObject)
                 {
-                    switch (reader.Value.ToString())
+                    switch (reader.Value?.ToString())
                     {
                         case "@version":
                             reader.ReadAndAssert();
@@ -2016,13 +2021,13 @@ namespace Newtonsoft.Json.Converters
 #if HAVE_XML_DOCUMENT_TYPE
         private void CreateDocumentType(JsonReader reader, IXmlDocument document, IXmlNode currentNode)
         {
-            string name = null;
-            string publicId = null;
-            string systemId = null;
-            string internalSubset = null;
+            string? name = null;
+            string? publicId = null;
+            string? systemId = null;
+            string? internalSubset = null;
             while (reader.Read() && reader.TokenType != JsonToken.EndObject)
             {
-                switch (reader.Value.ToString())
+                switch (reader.Value?.ToString())
                 {
                     case "@name":
                         reader.ReadAndAssert();
@@ -2050,7 +2055,7 @@ namespace Newtonsoft.Json.Converters
         }
 #endif
 
-        private IXmlElement CreateElement(string elementName, IXmlDocument document, string elementPrefix, XmlNamespaceManager manager)
+        private IXmlElement CreateElement(string elementName, IXmlDocument document, string? elementPrefix, XmlNamespaceManager manager)
         {
             string encodeName = EncodeSpecialCharacters ? XmlConvert.EncodeLocalName(elementName) : XmlConvert.EncodeName(elementName);
             string ns = string.IsNullOrEmpty(elementPrefix) ? manager.DefaultNamespace : manager.LookupNamespace(elementPrefix);
@@ -2072,7 +2077,7 @@ namespace Newtonsoft.Json.Converters
                             throw JsonSerializationException.Create(reader, "JSON root object has multiple properties. The root object must have a single property in order to create a valid XML document. Consider specifying a DeserializeRootElementName.");
                         }
 
-                        string propertyName = reader.Value.ToString();
+                        string propertyName = reader.Value!.ToString();
                         reader.ReadAndAssert();
 
                         if (reader.TokenType == JsonToken.StartArray)
@@ -2086,7 +2091,7 @@ namespace Newtonsoft.Json.Converters
 
                             if (count == 1 && WriteArrayAttribute)
                             {
-                                MiscellaneousUtils.GetQualifiedNameParts(propertyName, out string elementPrefix, out string localName);
+                                MiscellaneousUtils.GetQualifiedNameParts(propertyName, out string? elementPrefix, out string localName);
                                 string ns = string.IsNullOrEmpty(elementPrefix) ? manager.DefaultNamespace : manager.LookupNamespace(elementPrefix);
 
                                 foreach (IXmlNode childNode in currentNode.ChildNodes)
@@ -2105,7 +2110,7 @@ namespace Newtonsoft.Json.Converters
                         }
                         continue;
                     case JsonToken.StartConstructor:
-                        string constructorName = reader.Value.ToString();
+                        string constructorName = reader.Value!.ToString();
 
                         while (reader.Read() && reader.TokenType != JsonToken.EndConstructor)
                         {
@@ -2113,7 +2118,7 @@ namespace Newtonsoft.Json.Converters
                         }
                         break;
                     case JsonToken.Comment:
-                        currentNode.AppendChild(document.CreateComment((string)reader.Value));
+                        currentNode.AppendChild(document.CreateComment((string)reader.Value!));
                         break;
                     case JsonToken.EndObject:
                     case JsonToken.EndArray:
@@ -2131,7 +2136,7 @@ namespace Newtonsoft.Json.Converters
         /// <param name="attributeName">Attribute name to test.</param>
         /// <param name="prefix">The attribute name prefix if it has one, otherwise an empty string.</param>
         /// <returns><c>true</c> if attribute name is for a namespace attribute, otherwise <c>false</c>.</returns>
-        private bool IsNamespaceAttribute(string attributeName, out string prefix)
+        private bool IsNamespaceAttribute(string attributeName, [NotNullWhen(true)]out string? prefix)
         {
             if (attributeName.StartsWith("xmlns", StringComparison.Ordinal))
             {

Src/Newtonsoft.Json/DefaultJsonNameTable.cs

@@ -59,7 +59,7 @@ namespace Newtonsoft.Json
         /// <param name="start">The zero-based index into the array specifying the first character of the name.</param>
         /// <param name="length">The number of characters in the name.</param>
         /// <returns>A string containing the same characters as the specified range of characters in the given array.</returns>
-        public override string Get(char[] key, int start, int length)
+        public override string? Get(char[] key, int start, int length)
         {
             if (length == 0)
             {

Src/Newtonsoft.Json/IArrayPool.cs

@@ -17,6 +17,6 @@
         /// Return an array to the pool.
         /// </summary>
         /// <param name="array">The array that is being returned.</param>
-        void Return(T[] array);
+        void Return(T[]? array);
     }
 }

Src/Newtonsoft.Json/JsonContainerAttribute.cs

@@ -38,25 +38,25 @@ namespace Newtonsoft.Json
         /// Gets or sets the id.
         /// </summary>
         /// <value>The id.</value>
-        public string Id { get; set; }
+        public string? Id { get; set; }
 
         /// <summary>
         /// Gets or sets the title.
         /// </summary>
         /// <value>The title.</value>
-        public string Title { get; set; }
+        public string? Title { get; set; }
 
         /// <summary>
         /// Gets or sets the description.
         /// </summary>
         /// <value>The description.</value>
-        public string Description { get; set; }
+        public string? Description { get; set; }
 
         /// <summary>
         /// Gets or sets the collection's items converter.
         /// </summary>
         /// <value>The collection's items converter.</value>
-        public Type ItemConverterType { get; set; }
+        public Type? ItemConverterType { get; set; }
 
         /// <summary>
         /// The parameter list to use when constructing the <see cref="JsonConverter"/> described by <see cref="ItemConverterType"/>.
@@ -69,13 +69,13 @@ namespace Newtonsoft.Json
         /// [JsonContainer(ItemConverterType = typeof(MyContainerConverter), ItemConverterParameters = new object[] { 123, "Four" })]
         /// </code>
         /// </example>
-        public object[] ItemConverterParameters { get; set; }
+        public object[]? ItemConverterParameters { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="Type"/> of the <see cref="NamingStrategy"/>.
         /// </summary>
         /// <value>The <see cref="Type"/> of the <see cref="NamingStrategy"/>.</value>
-        public Type NamingStrategyType
+        public Type? NamingStrategyType
         {
             get => _namingStrategyType;
             set
@@ -96,7 +96,7 @@ namespace Newtonsoft.Json
         /// [JsonContainer(NamingStrategyType = typeof(MyNamingStrategy), NamingStrategyParameters = new object[] { 123, "Four" })]
         /// </code>
         /// </example>
-        public object[] NamingStrategyParameters
+        public object[]? NamingStrategyParameters
         {
             get => _namingStrategyParameters;
             set
@@ -106,7 +106,7 @@ namespace Newtonsoft.Json
             }
         }
 
-        internal NamingStrategy NamingStrategyInstance { get; set; }
+        internal NamingStrategy? NamingStrategyInstance { get; set; }
 
         // yuck. can't set nullable properties on an attribute in C#
         // have to use this approach to get an unset default state
@@ -114,8 +114,8 @@ namespace Newtonsoft.Json
         internal bool? _itemIsReference;
         internal ReferenceLoopHandling? _itemReferenceLoopHandling;
         internal TypeNameHandling? _itemTypeNameHandling;
-        private Type _namingStrategyType;
-        private object[] _namingStrategyParameters;
+        private Type? _namingStrategyType;
+        private object[]? _namingStrategyParameters;
 
         /// <summary>
         /// Gets or sets a value that indicates whether to preserve object references.

Src/Newtonsoft.Json/JsonConvert.cs

@@ -36,9 +36,10 @@ using Newtonsoft.Json.Converters;
 using Newtonsoft.Json.Serialization;
 using System.Text;
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 #if HAVE_XLINQ
 using System.Xml.Linq;
-
 #endif
 
 namespace Newtonsoft.Json
@@ -395,7 +396,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="value">The value to convert.</param>
         /// <returns>A JSON string representation of the <see cref="Uri"/>.</returns>
-        public static string ToString(Uri value)
+        public static string ToString(Uri? value)
         {
             if (value == null)
             {
@@ -415,7 +416,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="value">The value to convert.</param>
         /// <returns>A JSON string representation of the <see cref="String"/>.</returns>
-        public static string ToString(string value)
+        public static string ToString(string? value)
         {
             return ToString(value, '"');
         }
@@ -426,7 +427,7 @@ namespace Newtonsoft.Json
         /// <param name="value">The value to convert.</param>
         /// <param name="delimiter">The string delimiter character.</param>
         /// <returns>A JSON string representation of the <see cref="String"/>.</returns>
-        public static string ToString(string value, char delimiter)
+        public static string ToString(string? value, char delimiter)
         {
             return ToString(value, delimiter, StringEscapeHandling.Default);
         }
@@ -438,7 +439,7 @@ namespace Newtonsoft.Json
         /// <param name="delimiter">The string delimiter character.</param>
         /// <param name="stringEscapeHandling">The string escape handling.</param>
         /// <returns>A JSON string representation of the <see cref="String"/>.</returns>
-        public static string ToString(string value, char delimiter, StringEscapeHandling stringEscapeHandling)
+        public static string ToString(string? value, char delimiter, StringEscapeHandling stringEscapeHandling)
         {
             if (delimiter != '"' && delimiter != '\'')
             {
@@ -453,7 +454,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="value">The value to convert.</param>
         /// <returns>A JSON string representation of the <see cref="Object"/>.</returns>
-        public static string ToString(object value)
+        public static string ToString(object? value)
         {
             if (value == null)
             {
@@ -524,9 +525,9 @@ namespace Newtonsoft.Json
         /// <param name="value">The object to serialize.</param>
         /// <returns>A JSON string representation of the object.</returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value)
+        public static string SerializeObject(object? value)
         {
-            return SerializeObject(value, null, (JsonSerializerSettings)null);
+            return SerializeObject(value, null, (JsonSerializerSettings?)null);
         }
 
         /// <summary>
@@ -538,9 +539,9 @@ namespace Newtonsoft.Json
         /// A JSON string representation of the object.
         /// </returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, Formatting formatting)
+        public static string SerializeObject(object? value, Formatting formatting)
         {
-            return SerializeObject(value, formatting, (JsonSerializerSettings)null);
+            return SerializeObject(value, formatting, (JsonSerializerSettings?)null);
         }
 
         /// <summary>
@@ -550,9 +551,9 @@ namespace Newtonsoft.Json
         /// <param name="converters">A collection of converters used while serializing.</param>
         /// <returns>A JSON string representation of the object.</returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, params JsonConverter[] converters)
+        public static string SerializeObject(object? value, params JsonConverter[] converters)
         {
-            JsonSerializerSettings settings = (converters != null && converters.Length > 0)
+            JsonSerializerSettings? settings = (converters != null && converters.Length > 0)
                 ? new JsonSerializerSettings { Converters = converters }
                 : null;
 
@@ -567,9 +568,9 @@ namespace Newtonsoft.Json
         /// <param name="converters">A collection of converters used while serializing.</param>
         /// <returns>A JSON string representation of the object.</returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, Formatting formatting, params JsonConverter[] converters)
+        public static string SerializeObject(object? value, Formatting formatting, params JsonConverter[] converters)
         {
-            JsonSerializerSettings settings = (converters != null && converters.Length > 0)
+            JsonSerializerSettings? settings = (converters != null && converters.Length > 0)
                 ? new JsonSerializerSettings { Converters = converters }
                 : null;
 
@@ -586,7 +587,7 @@ namespace Newtonsoft.Json
         /// A JSON string representation of the object.
         /// </returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, JsonSerializerSettings settings)
+        public static string SerializeObject(object? value, JsonSerializerSettings settings)
         {
             return SerializeObject(value, null, settings);
         }
@@ -606,7 +607,7 @@ namespace Newtonsoft.Json
         /// A JSON string representation of the object.
         /// </returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, Type type, JsonSerializerSettings settings)
+        public static string SerializeObject(object? value, Type? type, JsonSerializerSettings? settings)
         {
             JsonSerializer jsonSerializer = JsonSerializer.CreateDefault(settings);
 
@@ -624,7 +625,7 @@ namespace Newtonsoft.Json
         /// A JSON string representation of the object.
         /// </returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, Formatting formatting, JsonSerializerSettings settings)
+        public static string SerializeObject(object? value, Formatting formatting, JsonSerializerSettings? settings)
         {
             return SerializeObject(value, null, formatting, settings);
         }
@@ -645,7 +646,7 @@ namespace Newtonsoft.Json
         /// A JSON string representation of the object.
         /// </returns>
         [DebuggerStepThrough]
-        public static string SerializeObject(object value, Type type, Formatting formatting, JsonSerializerSettings settings)
+        public static string SerializeObject(object? value, Type? type, Formatting formatting, JsonSerializerSettings? settings)
         {
             JsonSerializer jsonSerializer = JsonSerializer.CreateDefault(settings);
             jsonSerializer.Formatting = formatting;
@@ -653,7 +654,7 @@ namespace Newtonsoft.Json
             return SerializeObjectInternal(value, type, jsonSerializer);
         }
 
-        private static string SerializeObjectInternal(object value, Type type, JsonSerializer jsonSerializer)
+        private static string SerializeObjectInternal(object? value, Type? type, JsonSerializer jsonSerializer)
         {
             StringBuilder sb = new StringBuilder(256);
             StringWriter sw = new StringWriter(sb, CultureInfo.InvariantCulture);
@@ -675,9 +676,9 @@ namespace Newtonsoft.Json
         /// <param name="value">The JSON to deserialize.</param>
         /// <returns>The deserialized object from the JSON string.</returns>
         [DebuggerStepThrough]
-        public static object DeserializeObject(string value)
+        public static object? DeserializeObject(string value)
         {
-            return DeserializeObject(value, null, (JsonSerializerSettings)null);
+            return DeserializeObject(value, null, (JsonSerializerSettings?)null);
         }
 
         /// <summary>
@@ -690,7 +691,7 @@ namespace Newtonsoft.Json
         /// </param>
         /// <returns>The deserialized object from the JSON string.</returns>
         [DebuggerStepThrough]
-        public static object DeserializeObject(string value, JsonSerializerSettings settings)
+        public static object? DeserializeObject(string value, JsonSerializerSettings settings)
         {
             return DeserializeObject(value, null, settings);
         }
@@ -702,9 +703,9 @@ namespace Newtonsoft.Json
         /// <param name="type">The <see cref="Type"/> of object being deserialized.</param>
         /// <returns>The deserialized object from the JSON string.</returns>
         [DebuggerStepThrough]
-        public static object DeserializeObject(string value, Type type)
+        public static object? DeserializeObject(string value, Type type)
         {
-            return DeserializeObject(value, type, (JsonSerializerSettings)null);
+            return DeserializeObject(value, type, (JsonSerializerSettings?)null);
         }
 
         /// <summary>
@@ -716,7 +717,7 @@ namespace Newtonsoft.Json
         [DebuggerStepThrough]
         public static T DeserializeObject<T>(string value)
         {
-            return DeserializeObject<T>(value, (JsonSerializerSettings)null);
+            return DeserializeObject<T>(value, (JsonSerializerSettings?)null);
         }
 
         /// <summary>
@@ -765,9 +766,12 @@ namespace Newtonsoft.Json
         /// <param name="converters">Converters to use while deserializing.</param>
         /// <returns>The deserialized object from the JSON string.</returns>
         [DebuggerStepThrough]
+        [return: MaybeNull]
         public static T DeserializeObject<T>(string value, params JsonConverter[] converters)
         {
+#pragma warning disable CS8601 // Possible null reference assignment.
             return (T)DeserializeObject(value, typeof(T), converters);
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -781,9 +785,12 @@ namespace Newtonsoft.Json
         /// </param>
         /// <returns>The deserialized object from the JSON string.</returns>
         [DebuggerStepThrough]
-        public static T DeserializeObject<T>(string value, JsonSerializerSettings settings)
+        [return: MaybeNull]
+        public static T DeserializeObject<T>(string value, JsonSerializerSettings? settings)
         {
+#pragma warning disable CS8601 // Possible null reference assignment.
             return (T)DeserializeObject(value, typeof(T), settings);
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -794,9 +801,9 @@ namespace Newtonsoft.Json
         /// <param name="converters">Converters to use while deserializing.</param>
         /// <returns>The deserialized object from the JSON string.</returns>
         [DebuggerStepThrough]
-        public static object DeserializeObject(string value, Type type, params JsonConverter[] converters)
+        public static object? DeserializeObject(string value, Type type, params JsonConverter[] converters)
         {
-            JsonSerializerSettings settings = (converters != null && converters.Length > 0)
+            JsonSerializerSettings? settings = (converters != null && converters.Length > 0)
                 ? new JsonSerializerSettings { Converters = converters }
                 : null;
 
@@ -813,7 +820,7 @@ namespace Newtonsoft.Json
         /// If this is <c>null</c>, default serialization settings will be used.
         /// </param>
         /// <returns>The deserialized object from the JSON string.</returns>
-        public static object DeserializeObject(string value, Type type, JsonSerializerSettings settings)
+        public static object? DeserializeObject(string value, Type? type, JsonSerializerSettings? settings)
         {
             ValidationUtils.ArgumentNotNull(value, nameof(value));
 
@@ -853,7 +860,7 @@ namespace Newtonsoft.Json
         /// The <see cref="JsonSerializerSettings"/> used to deserialize the object.
         /// If this is <c>null</c>, default serialization settings will be used.
         /// </param>
-        public static void PopulateObject(string value, object target, JsonSerializerSettings settings)
+        public static void PopulateObject(string value, object target, JsonSerializerSettings? settings)
         {
             JsonSerializer jsonSerializer = JsonSerializer.CreateDefault(settings);
 
@@ -882,7 +889,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="node">The node to serialize.</param>
         /// <returns>A JSON string of the <see cref="XmlNode"/>.</returns>
-        public static string SerializeXmlNode(XmlNode node)
+        public static string SerializeXmlNode(XmlNode? node)
         {
             return SerializeXmlNode(node, Formatting.None);
         }
@@ -893,7 +900,7 @@ namespace Newtonsoft.Json
         /// <param name="node">The node to serialize.</param>
         /// <param name="formatting">Indicates how the output should be formatted.</param>
         /// <returns>A JSON string of the <see cref="XmlNode"/>.</returns>
-        public static string SerializeXmlNode(XmlNode node, Formatting formatting)
+        public static string SerializeXmlNode(XmlNode? node, Formatting formatting)
         {
             XmlNodeConverter converter = new XmlNodeConverter();
 
@@ -907,7 +914,7 @@ namespace Newtonsoft.Json
         /// <param name="formatting">Indicates how the output should be formatted.</param>
         /// <param name="omitRootObject">Omits writing the root object.</param>
         /// <returns>A JSON string of the <see cref="XmlNode"/>.</returns>
-        public static string SerializeXmlNode(XmlNode node, Formatting formatting, bool omitRootObject)
+        public static string SerializeXmlNode(XmlNode? node, Formatting formatting, bool omitRootObject)
         {
             XmlNodeConverter converter = new XmlNodeConverter { OmitRootObject = omitRootObject };
 
@@ -919,7 +926,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="value">The JSON string.</param>
         /// <returns>The deserialized <see cref="XmlNode"/>.</returns>
-        public static XmlDocument DeserializeXmlNode(string value)
+        public static XmlDocument? DeserializeXmlNode(string value)
         {
             return DeserializeXmlNode(value, null);
         }
@@ -930,7 +937,7 @@ namespace Newtonsoft.Json
         /// <param name="value">The JSON string.</param>
         /// <param name="deserializeRootElementName">The name of the root element to append when deserializing.</param>
         /// <returns>The deserialized <see cref="XmlNode"/>.</returns>
-        public static XmlDocument DeserializeXmlNode(string value, string deserializeRootElementName)
+        public static XmlDocument? DeserializeXmlNode(string value, string? deserializeRootElementName)
         {
             return DeserializeXmlNode(value, deserializeRootElementName, false);
         }
@@ -946,7 +953,7 @@ namespace Newtonsoft.Json
         /// This attribute helps preserve arrays when converting the written XML back to JSON.
         /// </param>
         /// <returns>The deserialized <see cref="XmlNode"/>.</returns>
-        public static XmlDocument DeserializeXmlNode(string value, string deserializeRootElementName, bool writeArrayAttribute)
+        public static XmlDocument? DeserializeXmlNode(string value, string? deserializeRootElementName, bool writeArrayAttribute)
         {
             return DeserializeXmlNode(value, deserializeRootElementName, writeArrayAttribute, false);
         }
@@ -968,14 +975,14 @@ namespace Newtonsoft.Json
         /// as part of the XML element name.
         /// </param>
         /// <returns>The deserialized <see cref="XmlNode"/>.</returns>
-        public static XmlDocument DeserializeXmlNode(string value, string deserializeRootElementName, bool writeArrayAttribute, bool encodeSpecialCharacters)
+        public static XmlDocument? DeserializeXmlNode(string value, string? deserializeRootElementName, bool writeArrayAttribute, bool encodeSpecialCharacters)
         {
             XmlNodeConverter converter = new XmlNodeConverter();
             converter.DeserializeRootElementName = deserializeRootElementName;
             converter.WriteArrayAttribute = writeArrayAttribute;
             converter.EncodeSpecialCharacters = encodeSpecialCharacters;
 
-            return (XmlDocument)DeserializeObject(value, typeof(XmlDocument), converter);
+            return (XmlDocument?)DeserializeObject(value, typeof(XmlDocument), converter);
         }
 #endif
 
@@ -985,7 +992,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="node">The node to convert to JSON.</param>
         /// <returns>A JSON string of the <see cref="XNode"/>.</returns>
-        public static string SerializeXNode(XObject node)
+        public static string SerializeXNode(XObject? node)
         {
             return SerializeXNode(node, Formatting.None);
         }
@@ -996,7 +1003,7 @@ namespace Newtonsoft.Json
         /// <param name="node">The node to convert to JSON.</param>
         /// <param name="formatting">Indicates how the output should be formatted.</param>
         /// <returns>A JSON string of the <see cref="XNode"/>.</returns>
-        public static string SerializeXNode(XObject node, Formatting formatting)
+        public static string SerializeXNode(XObject? node, Formatting formatting)
         {
             return SerializeXNode(node, formatting, false);
         }
@@ -1008,7 +1015,7 @@ namespace Newtonsoft.Json
         /// <param name="formatting">Indicates how the output should be formatted.</param>
         /// <param name="omitRootObject">Omits writing the root object.</param>
         /// <returns>A JSON string of the <see cref="XNode"/>.</returns>
-        public static string SerializeXNode(XObject node, Formatting formatting, bool omitRootObject)
+        public static string SerializeXNode(XObject? node, Formatting formatting, bool omitRootObject)
         {
             XmlNodeConverter converter = new XmlNodeConverter { OmitRootObject = omitRootObject };
 
@@ -1020,7 +1027,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="value">The JSON string.</param>
         /// <returns>The deserialized <see cref="XNode"/>.</returns>
-        public static XDocument DeserializeXNode(string value)
+        public static XDocument? DeserializeXNode(string value)
         {
             return DeserializeXNode(value, null);
         }
@@ -1031,7 +1038,7 @@ namespace Newtonsoft.Json
         /// <param name="value">The JSON string.</param>
         /// <param name="deserializeRootElementName">The name of the root element to append when deserializing.</param>
         /// <returns>The deserialized <see cref="XNode"/>.</returns>
-        public static XDocument DeserializeXNode(string value, string deserializeRootElementName)
+        public static XDocument? DeserializeXNode(string value, string? deserializeRootElementName)
         {
             return DeserializeXNode(value, deserializeRootElementName, false);
         }
@@ -1047,7 +1054,7 @@ namespace Newtonsoft.Json
         /// This attribute helps preserve arrays when converting the written XML back to JSON.
         /// </param>
         /// <returns>The deserialized <see cref="XNode"/>.</returns>
-        public static XDocument DeserializeXNode(string value, string deserializeRootElementName, bool writeArrayAttribute)
+        public static XDocument? DeserializeXNode(string value, string? deserializeRootElementName, bool writeArrayAttribute)
         {
             return DeserializeXNode(value, deserializeRootElementName, writeArrayAttribute, false);
         }
@@ -1069,14 +1076,14 @@ namespace Newtonsoft.Json
         /// as part of the XML element name.
         /// </param>
         /// <returns>The deserialized <see cref="XNode"/>.</returns>
-        public static XDocument DeserializeXNode(string value, string deserializeRootElementName, bool writeArrayAttribute, bool encodeSpecialCharacters)
+        public static XDocument? DeserializeXNode(string value, string? deserializeRootElementName, bool writeArrayAttribute, bool encodeSpecialCharacters)
         {
             XmlNodeConverter converter = new XmlNodeConverter();
             converter.DeserializeRootElementName = deserializeRootElementName;
             converter.WriteArrayAttribute = writeArrayAttribute;
             converter.EncodeSpecialCharacters = encodeSpecialCharacters;
 
-            return (XDocument)DeserializeObject(value, typeof(XDocument), converter);
+            return (XDocument?)DeserializeObject(value, typeof(XDocument), converter);
         }
 #endif
         #endregion

Src/Newtonsoft.Json/JsonConverter.cs

@@ -26,6 +26,8 @@
 using System;
 using Newtonsoft.Json.Utilities;
 using System.Globalization;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Newtonsoft.Json
 {
@@ -40,7 +42,7 @@ namespace Newtonsoft.Json
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public abstract void WriteJson(JsonWriter writer, object value, JsonSerializer serializer);
+        public abstract void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer);
 
         /// <summary>
         /// Reads the JSON representation of the object.
@@ -50,7 +52,7 @@ namespace Newtonsoft.Json
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public abstract object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer);
+        public abstract object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer);
 
         /// <summary>
         /// Determines whether this instance can convert the specified object type.
@@ -86,13 +88,15 @@ namespace Newtonsoft.Json
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public sealed override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        public sealed override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {
             if (!(value != null ? value is T : ReflectionUtils.IsNullable(typeof(T))))
             {
                 throw new JsonSerializationException("Converter cannot write specified value to JSON. {0} is required.".FormatWith(CultureInfo.InvariantCulture, typeof(T)));
             }
+#pragma warning disable CS8601 // Possible null reference assignment.
             WriteJson(writer, (T)value, serializer);
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -101,7 +105,7 @@ namespace Newtonsoft.Json
         /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
-        public abstract void WriteJson(JsonWriter writer, T value, JsonSerializer serializer);
+        public abstract void WriteJson(JsonWriter writer, [AllowNull]T value, JsonSerializer serializer);
 
         /// <summary>
         /// Reads the JSON representation of the object.
@@ -111,14 +115,16 @@ namespace Newtonsoft.Json
         /// <param name="existingValue">The existing value of object being read.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public sealed override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        public sealed override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
         {
             bool existingIsNull = existingValue == null;
             if (!(existingIsNull || existingValue is T))
             {
                 throw new JsonSerializationException("Converter cannot read JSON with the specified existing value. {0} is required.".FormatWith(CultureInfo.InvariantCulture, typeof(T)));
             }
+#pragma warning disable CS8601 // Possible null reference assignment.
             return ReadJson(reader, objectType, existingIsNull ? default : (T)existingValue, !existingIsNull, serializer);
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -130,7 +136,7 @@ namespace Newtonsoft.Json
         /// <param name="hasExistingValue">The existing value has a value.</param>
         /// <param name="serializer">The calling serializer.</param>
         /// <returns>The object value.</returns>
-        public abstract T ReadJson(JsonReader reader, Type objectType, T existingValue, bool hasExistingValue, JsonSerializer serializer);
+        public abstract T ReadJson(JsonReader reader, Type objectType, [AllowNull]T existingValue, bool hasExistingValue, JsonSerializer serializer);
 
         /// <summary>
         /// Determines whether this instance can convert the specified object type.

Src/Newtonsoft.Json/JsonConverterAttribute.cs

@@ -47,7 +47,7 @@ namespace Newtonsoft.Json
         /// The parameter list to use when constructing the <see cref="JsonConverter"/> described by <see cref="ConverterType"/>.
         /// If <c>null</c>, the default constructor is used.
         /// </summary>
-        public object[] ConverterParameters { get; }
+        public object[]? ConverterParameters { get; }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonConverterAttribute"/> class.

Src/Newtonsoft.Json/JsonException.cs

@@ -63,7 +63,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception, or <c>null</c> if no inner exception is specified.</param>
-        public JsonException(string message, Exception innerException)
+        public JsonException(string message, Exception? innerException)
             : base(message, innerException)
         {
         }

Src/Newtonsoft.Json/JsonNameTable.cs

@@ -12,6 +12,6 @@
         /// <param name="start">The zero-based index into the array specifying the first character of the name.</param>
         /// <param name="length">The number of characters in the name.</param>
         /// <returns>A string containing the same characters as the specified range of characters in the given array.</returns>
-        public abstract string Get(char[] key, int start, int length);
+        public abstract string? Get(char[] key, int start, int length);
     }
 }

Src/Newtonsoft.Json/JsonPosition.cs

@@ -46,7 +46,7 @@ namespace Newtonsoft.Json
 
         internal JsonContainerType Type;
         internal int Position;
-        internal string PropertyName;
+        internal string? PropertyName;
         internal bool HasIndex;
 
         public JsonPosition(JsonContainerType type)
@@ -62,7 +62,7 @@ namespace Newtonsoft.Json
             switch (Type)
             {
                 case JsonContainerType.Object:
-                    return PropertyName.Length + 5;
+                    return PropertyName!.Length + 5;
                 case JsonContainerType.Array:
                 case JsonContainerType.Constructor:
                     return MathUtils.IntLength((ulong)Position) + 2;
@@ -71,12 +71,12 @@ namespace Newtonsoft.Json
             }
         }
 
-        internal void WriteTo(StringBuilder sb, ref StringWriter writer, ref char[] buffer)
+        internal void WriteTo(StringBuilder sb, ref StringWriter? writer, ref char[]? buffer)
         {
             switch (Type)
             {
                 case JsonContainerType.Object:
-                    string propertyName = PropertyName;
+                    string propertyName = PropertyName!;
                     if (propertyName.IndexOfAny(SpecialCharacters) != -1)
                     {
                         sb.Append(@"['");
@@ -130,8 +130,8 @@ namespace Newtonsoft.Json
             }
 
             StringBuilder sb = new StringBuilder(capacity);
-            StringWriter writer = null;
-            char[] buffer = null;
+            StringWriter? writer = null;
+            char[]? buffer = null;
             if (positions != null)
             {
                 foreach (JsonPosition state in positions)
@@ -147,7 +147,7 @@ namespace Newtonsoft.Json
             return sb.ToString();
         }
 
-        internal static string FormatMessage(IJsonLineInfo lineInfo, string path, string message)
+        internal static string FormatMessage(IJsonLineInfo? lineInfo, string path, string message)
         {
             // don't add a fullstop and space when message ends with a new line
             if (!message.EndsWith(Environment.NewLine, StringComparison.Ordinal))

Src/Newtonsoft.Json/JsonPropertyAttribute.cs

@@ -52,7 +52,7 @@ namespace Newtonsoft.Json
         /// Gets or sets the <see cref="JsonConverter"/> type used when serializing the property's collection items.
         /// </summary>
         /// <value>The collection's items <see cref="JsonConverter"/> type.</value>
-        public Type ItemConverterType { get; set; }
+        public Type? ItemConverterType { get; set; }
 
         /// <summary>
         /// The parameter list to use when constructing the <see cref="JsonConverter"/> described by <see cref="ItemConverterType"/>.
@@ -65,13 +65,13 @@ namespace Newtonsoft.Json
         /// [JsonProperty(ItemConverterType = typeof(MyContainerConverter), ItemConverterParameters = new object[] { 123, "Four" })]
         /// </code>
         /// </example>
-        public object[] ItemConverterParameters { get; set; }
+        public object[]? ItemConverterParameters { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="Type"/> of the <see cref="NamingStrategy"/>.
         /// </summary>
         /// <value>The <see cref="Type"/> of the <see cref="NamingStrategy"/>.</value>
-        public Type NamingStrategyType { get; set; }
+        public Type? NamingStrategyType { get; set; }
 
         /// <summary>
         /// The parameter list to use when constructing the <see cref="NamingStrategy"/> described by <see cref="JsonPropertyAttribute.NamingStrategyType"/>.
@@ -84,7 +84,7 @@ namespace Newtonsoft.Json
         /// [JsonProperty(NamingStrategyType = typeof(MyNamingStrategy), NamingStrategyParameters = new object[] { 123, "Four" })]
         /// </code>
         /// </example>
-        public object[] NamingStrategyParameters { get; set; }
+        public object[]? NamingStrategyParameters { get; set; }
 
         /// <summary>
         /// Gets or sets the null value handling used when serializing this property.
@@ -172,7 +172,7 @@ namespace Newtonsoft.Json
         /// Gets or sets the name of the property.
         /// </summary>
         /// <value>The name of the property.</value>
-        public string PropertyName { get; set; }
+        public string? PropertyName { get; set; }
 
         /// <summary>
         /// Gets or sets the reference loop handling used when serializing the property's collection items.

Src/Newtonsoft.Json/JsonReader.Async.cs

@@ -101,12 +101,12 @@ namespace Newtonsoft.Json
         /// property returns the <see cref="byte"/>[]. This result will be <c>null</c> at the end of an array.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task<byte[]> ReadAsBytesAsync(CancellationToken cancellationToken = default)
+        public virtual Task<byte[]?> ReadAsBytesAsync(CancellationToken cancellationToken = default)
         {
-            return cancellationToken.CancelIfRequestedAsync<byte[]>() ?? Task.FromResult(ReadAsBytes());
+            return cancellationToken.CancelIfRequestedAsync<byte[]?>() ?? Task.FromResult(ReadAsBytes());
         }
 
-        internal async Task<byte[]> ReadArrayIntoByteArrayAsync(CancellationToken cancellationToken)
+        internal async Task<byte[]?> ReadArrayIntoByteArrayAsync(CancellationToken cancellationToken)
         {
             List<byte> buffer = new List<byte>();
 
@@ -199,9 +199,9 @@ namespace Newtonsoft.Json
         /// property returns the <see cref="string"/>. This result will be <c>null</c> at the end of an array.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task<string> ReadAsStringAsync(CancellationToken cancellationToken = default)
+        public virtual Task<string?> ReadAsStringAsync(CancellationToken cancellationToken = default)
         {
-            return cancellationToken.CancelIfRequestedAsync<string>() ?? Task.FromResult(ReadAsString());
+            return cancellationToken.CancelIfRequestedAsync<string?>() ?? Task.FromResult(ReadAsString());
         }
 
         internal async Task<bool> ReadAndMoveToContentAsync(CancellationToken cancellationToken)

Src/Newtonsoft.Json/JsonReader.cs

@@ -113,18 +113,18 @@ namespace Newtonsoft.Json
 
         // current Token data
         private JsonToken _tokenType;
-        private object _value;
+        private object? _value;
         internal char _quoteChar;
         internal State _currentState;
         private JsonPosition _currentPosition;
-        private CultureInfo _culture;
+        private CultureInfo? _culture;
         private DateTimeZoneHandling _dateTimeZoneHandling;
         private int? _maxDepth;
         private bool _hasExceededMaxDepth;
         internal DateParseHandling _dateParseHandling;
         internal FloatParseHandling _floatParseHandling;
-        private string _dateFormatString;
-        private List<JsonPosition> _stack;
+        private string? _dateFormatString;
+        private List<JsonPosition>? _stack;
 
         /// <summary>
         /// Gets the current reader state.
@@ -219,7 +219,7 @@ namespace Newtonsoft.Json
         /// <summary>
         /// Gets or sets how custom date formatted strings are parsed when reading JSON.
         /// </summary>
-        public string DateFormatString
+        public string? DateFormatString
         {
             get => _dateFormatString;
             set => _dateFormatString = value;
@@ -250,12 +250,12 @@ namespace Newtonsoft.Json
         /// <summary>
         /// Gets the text value of the current JSON token.
         /// </summary>
-        public virtual object Value => _value;
+        public virtual object? Value => _value;
 
         /// <summary>
         /// Gets the .NET type for the current JSON token.
         /// </summary>
-        public virtual Type ValueType => _value?.GetType();
+        public virtual Type? ValueType => _value?.GetType();
 
         /// <summary>
         /// Gets the depth of the current token in the JSON document.
@@ -295,7 +295,7 @@ namespace Newtonsoft.Json
 
                 JsonPosition? current = insideContainer ? (JsonPosition?)_currentPosition : null;
 
-                return JsonPosition.BuildPath(_stack, current);
+                return JsonPosition.BuildPath(_stack!, current);
             }
         }
 
@@ -408,7 +408,7 @@ namespace Newtonsoft.Json
                     return null;
                 case JsonToken.Integer:
                 case JsonToken.Float:
-                    object v = Value;
+                    object v = Value!;
                     if (v is int i)
                     {
                         return i;
@@ -436,14 +436,14 @@ namespace Newtonsoft.Json
                     SetToken(JsonToken.Integer, i, false);
                     return i;
                 case JsonToken.String:
-                    string s = (string)Value;
+                    string? s = (string?)Value;
                     return ReadInt32String(s);
             }
 
             throw JsonReaderException.Create(this, "Error reading integer. Unexpected token: {0}.".FormatWith(CultureInfo.InvariantCulture, t));
         }
 
-        internal int? ReadInt32String(string s)
+        internal int? ReadInt32String(string? s)
         {
             if (string.IsNullOrEmpty(s))
             {
@@ -467,7 +467,7 @@ namespace Newtonsoft.Json
         /// Reads the next JSON token from the source as a <see cref="String"/>.
         /// </summary>
         /// <returns>A <see cref="String"/>. This method will return <c>null</c> at the end of an array.</returns>
-        public virtual string ReadAsString()
+        public virtual string? ReadAsString()
         {
             JsonToken t = GetContentToken();
 
@@ -478,12 +478,12 @@ namespace Newtonsoft.Json
                 case JsonToken.EndArray:
                     return null;
                 case JsonToken.String:
-                    return (string)Value;
+                    return (string?)Value;
             }
 
             if (JsonTokenUtils.IsPrimitiveToken(t))
             {
-                object v = Value;
+                object? v = Value;
                 if (v != null)
                 {
                     string s;
@@ -508,7 +508,7 @@ namespace Newtonsoft.Json
         /// Reads the next JSON token from the source as a <see cref="Byte"/>[].
         /// </summary>
         /// <returns>A <see cref="Byte"/>[] or <c>null</c> if the next JSON token is null. This method will return <c>null</c> at the end of an array.</returns>
-        public virtual byte[] ReadAsBytes()
+        public virtual byte[]? ReadAsBytes()
         {
             JsonToken t = GetContentToken();
 
@@ -518,7 +518,7 @@ namespace Newtonsoft.Json
                 {
                     ReadIntoWrappedTypeObject();
 
-                    byte[] data = ReadAsBytes();
+                    byte[]? data = ReadAsBytes();
                     ReaderReadAndAssert();
 
                     if (TokenType != JsonToken.EndObject)
@@ -533,7 +533,7 @@ namespace Newtonsoft.Json
                 {
                     // attempt to convert possible base 64 or GUID string to bytes
                     // GUID has to have format 00000000-0000-0000-0000-000000000000
-                    string s = (string)Value;
+                    string s = (string)Value!;
 
                     byte[] data;
 
@@ -565,7 +565,7 @@ namespace Newtonsoft.Json
                         return data;
                     }
 
-                    return (byte[])Value;
+                    return (byte[]?)Value;
                 case JsonToken.StartArray:
                     return ReadArrayIntoByteArray();
             }
@@ -627,7 +627,7 @@ namespace Newtonsoft.Json
                     return null;
                 case JsonToken.Integer:
                 case JsonToken.Float:
-                    object v = Value;
+                    object v = Value!;
                     if (v is double d)
                     {
                         return d;
@@ -648,13 +648,13 @@ namespace Newtonsoft.Json
 
                     return (double)d;
                 case JsonToken.String:
-                    return ReadDoubleString((string)Value);
+                    return ReadDoubleString((string?)Value);
             }
 
             throw JsonReaderException.Create(this, "Error reading double. Unexpected token: {0}.".FormatWith(CultureInfo.InvariantCulture, t));
         }
 
-        internal double? ReadDoubleString(string s)
+        internal double? ReadDoubleString(string? s)
         {
             if (string.IsNullOrEmpty(s))
             {
@@ -705,15 +705,15 @@ namespace Newtonsoft.Json
                     SetToken(JsonToken.Boolean, b, false);
                     return b;
                 case JsonToken.String:
-                    return ReadBooleanString((string)Value);
+                    return ReadBooleanString((string?)Value);
                 case JsonToken.Boolean:
-                    return (bool)Value;
+                    return (bool)Value!;
             }
 
             throw JsonReaderException.Create(this, "Error reading boolean. Unexpected token: {0}.".FormatWith(CultureInfo.InvariantCulture, t));
         }
 
-        internal bool? ReadBooleanString(string s)
+        internal bool? ReadBooleanString(string? s)
         {
             if (string.IsNullOrEmpty(s))
             {
@@ -749,7 +749,7 @@ namespace Newtonsoft.Json
                     return null;
                 case JsonToken.Integer:
                 case JsonToken.Float:
-                    object v = Value;
+                    object v = Value!;
                     
                     if (v is decimal d)
                     {
@@ -778,13 +778,13 @@ namespace Newtonsoft.Json
                     SetToken(JsonToken.Float, d, false);
                     return d;
                 case JsonToken.String:
-                    return ReadDecimalString((string)Value);
+                    return ReadDecimalString((string?)Value);
             }
 
             throw JsonReaderException.Create(this, "Error reading decimal. Unexpected token: {0}.".FormatWith(CultureInfo.InvariantCulture, t));
         }
 
-        internal decimal? ReadDecimalString(string s)
+        internal decimal? ReadDecimalString(string? s)
         {
             if (string.IsNullOrEmpty(s))
             {
@@ -830,16 +830,15 @@ namespace Newtonsoft.Json
                     }
 #endif
 
-                    return (DateTime)Value;
+                    return (DateTime)Value!;
                 case JsonToken.String:
-                    string s = (string)Value;
-                    return ReadDateTimeString(s);
+                    return ReadDateTimeString((string?)Value);
             }
 
             throw JsonReaderException.Create(this, "Error reading date. Unexpected token: {0}.".FormatWith(CultureInfo.InvariantCulture, TokenType));
         }
 
-        internal DateTime? ReadDateTimeString(string s)
+        internal DateTime? ReadDateTimeString(string? s)
         {
             if (string.IsNullOrEmpty(s))
             {
@@ -885,16 +884,16 @@ namespace Newtonsoft.Json
                         SetToken(JsonToken.Date, new DateTimeOffset(time), false);
                     }
 
-                    return (DateTimeOffset)Value;
+                    return (DateTimeOffset)Value!;
                 case JsonToken.String:
-                    string s = (string)Value;
+                    string? s = (string?)Value;
                     return ReadDateTimeOffsetString(s);
                 default:
                     throw JsonReaderException.Create(this, "Error reading date. Unexpected token: {0}.".FormatWith(CultureInfo.InvariantCulture, t));
             }
         }
 
-        internal DateTimeOffset? ReadDateTimeOffsetString(string s)
+        internal DateTimeOffset? ReadDateTimeOffsetString(string? s)
         {
             if (string.IsNullOrEmpty(s))
             {
@@ -985,7 +984,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="newToken">The new token.</param>
         /// <param name="value">The value.</param>
-        protected void SetToken(JsonToken newToken, object value)
+        protected void SetToken(JsonToken newToken, object? value)
         {
             SetToken(newToken, value, true);
         }
@@ -996,7 +995,7 @@ namespace Newtonsoft.Json
         /// <param name="newToken">The new token.</param>
         /// <param name="value">The value.</param>
         /// <param name="updateIndex">A flag indicating whether the position index inside an array should be updated.</param>
-        protected void SetToken(JsonToken newToken, object value, bool updateIndex)
+        protected void SetToken(JsonToken newToken, object? value, bool updateIndex)
         {
             _tokenType = newToken;
             _value = value;
@@ -1027,7 +1026,7 @@ namespace Newtonsoft.Json
                 case JsonToken.PropertyName:
                     _currentState = State.Property;
 
-                    _currentPosition.PropertyName = (string)value;
+                    _currentPosition.PropertyName = (string)value!;
                     break;
                 case JsonToken.Undefined:
                 case JsonToken.Integer:
@@ -1170,7 +1169,7 @@ namespace Newtonsoft.Json
             }
         }
 
-        internal void ReadForTypeAndAssert(JsonContract contract, bool hasConverter)
+        internal void ReadForTypeAndAssert(JsonContract? contract, bool hasConverter)
         {
             if (!ReadForType(contract, hasConverter))
             {
@@ -1178,7 +1177,7 @@ namespace Newtonsoft.Json
             }
         }
 
-        internal bool ReadForType(JsonContract contract, bool hasConverter)
+        internal bool ReadForType(JsonContract? contract, bool hasConverter)
         {
             // don't read properties with converters as a specific value
             // the value might be a string which will then get converted which will error if read as date for example

Src/Newtonsoft.Json/JsonReaderException.cs

@@ -54,7 +54,7 @@ namespace Newtonsoft.Json
         /// Gets the path to the JSON where the error occurred.
         /// </summary>
         /// <value>The path to the JSON where the error occurred.</value>
-        public string Path { get; }
+        public string? Path { get; }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonReaderException"/> class.
@@ -107,7 +107,7 @@ namespace Newtonsoft.Json
         /// <param name="lineNumber">The line number indicating where the error occurred.</param>
         /// <param name="linePosition">The line position indicating where the error occurred.</param>
         /// <param name="innerException">The exception that is the cause of the current exception, or <c>null</c> if no inner exception is specified.</param>
-        public JsonReaderException(string message, string path, int lineNumber, int linePosition, Exception innerException)
+        public JsonReaderException(string message, string path, int lineNumber, int linePosition, Exception? innerException)
             : base(message, innerException)
         {
             Path = path;
@@ -120,12 +120,12 @@ namespace Newtonsoft.Json
             return Create(reader, message, null);
         }
 
-        internal static JsonReaderException Create(JsonReader reader, string message, Exception ex)
+        internal static JsonReaderException Create(JsonReader reader, string message, Exception? ex)
         {
             return Create(reader as IJsonLineInfo, reader.Path, message, ex);
         }
 
-        internal static JsonReaderException Create(IJsonLineInfo lineInfo, string path, string message, Exception ex)
+        internal static JsonReaderException Create(IJsonLineInfo? lineInfo, string path, string message, Exception? ex)
         {
             message = JsonPosition.FormatMessage(lineInfo, path, message);
 

Src/Newtonsoft.Json/JsonSerializationException.cs

@@ -54,7 +54,7 @@ namespace Newtonsoft.Json
         /// Gets the path to the JSON where the error occurred.
         /// </summary>
         /// <value>The path to the JSON where the error occurred.</value>
-        public string Path { get; }
+        public string? Path { get; }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonSerializationException"/> class.
@@ -107,7 +107,7 @@ namespace Newtonsoft.Json
         /// <param name="lineNumber">The line number indicating where the error occurred.</param>
         /// <param name="linePosition">The line position indicating where the error occurred.</param>
         /// <param name="innerException">The exception that is the cause of the current exception, or <c>null</c> if no inner exception is specified.</param>
-        public JsonSerializationException(string message, string path, int lineNumber, int linePosition, Exception innerException)
+        public JsonSerializationException(string message, string path, int lineNumber, int linePosition, Exception? innerException)
             : base(message, innerException)
         {
             Path = path;
@@ -120,12 +120,12 @@ namespace Newtonsoft.Json
             return Create(reader, message, null);
         }
 
-        internal static JsonSerializationException Create(JsonReader reader, string message, Exception ex)
+        internal static JsonSerializationException Create(JsonReader reader, string message, Exception? ex)
         {
             return Create(reader as IJsonLineInfo, reader.Path, message, ex);
         }
 
-        internal static JsonSerializationException Create(IJsonLineInfo lineInfo, string path, string message, Exception ex)
+        internal static JsonSerializationException Create(IJsonLineInfo? lineInfo, string path, string message, Exception? ex)
         {
             message = JsonPosition.FormatMessage(lineInfo, path, message);
 

Src/Newtonsoft.Json/JsonSerializer.cs

@@ -35,6 +35,8 @@ using Newtonsoft.Json.Serialization;
 using Newtonsoft.Json.Utilities;
 using System.Runtime.Serialization;
 using ErrorEventArgs = Newtonsoft.Json.Serialization.ErrorEventArgs;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Newtonsoft.Json
 {
@@ -54,13 +56,13 @@ namespace Newtonsoft.Json
         internal DefaultValueHandling _defaultValueHandling;
         internal ConstructorHandling _constructorHandling;
         internal MetadataPropertyHandling _metadataPropertyHandling;
-        internal JsonConverterCollection _converters;
+        internal JsonConverterCollection? _converters;
         internal IContractResolver _contractResolver;
-        internal ITraceWriter _traceWriter;
-        internal IEqualityComparer _equalityComparer;
+        internal ITraceWriter? _traceWriter;
+        internal IEqualityComparer? _equalityComparer;
         internal ISerializationBinder _serializationBinder;
         internal StreamingContext _context;
-        private IReferenceResolver _referenceResolver;
+        private IReferenceResolver? _referenceResolver;
 
         private Formatting? _formatting;
         private DateFormatHandling? _dateFormatHandling;
@@ -73,7 +75,7 @@ namespace Newtonsoft.Json
         private int? _maxDepth;
         private bool _maxDepthSet;
         private bool? _checkAdditionalContent;
-        private string _dateFormatString;
+        private string? _dateFormatString;
         private bool _dateFormatStringSet;
 
         /// <summary>
@@ -84,7 +86,7 @@ namespace Newtonsoft.Json
         /// <summary>
         /// Gets or sets the <see cref="IReferenceResolver"/> used by the serializer when resolving references.
         /// </summary>
-        public virtual IReferenceResolver ReferenceResolver
+        public virtual IReferenceResolver? ReferenceResolver
         {
             get => GetReferenceResolver();
             set
@@ -106,11 +108,6 @@ namespace Newtonsoft.Json
         {
             get
             {
-                if (_serializationBinder == null)
-                {
-                    return null;
-                }
-
                 if (_serializationBinder is SerializationBinder legacySerializationBinder)
                 {
                     return legacySerializationBinder;
@@ -155,7 +152,7 @@ namespace Newtonsoft.Json
         /// Gets or sets the <see cref="ITraceWriter"/> used by the serializer when writing trace messages.
         /// </summary>
         /// <value>The trace writer.</value>
-        public virtual ITraceWriter TraceWriter
+        public virtual ITraceWriter? TraceWriter
         {
             get => _traceWriter;
             set => _traceWriter = value;
@@ -165,7 +162,7 @@ namespace Newtonsoft.Json
         /// Gets or sets the equality comparer used by the serializer when comparing references.
         /// </summary>
         /// <value>The equality comparer.</value>
-        public virtual IEqualityComparer EqualityComparer
+        public virtual IEqualityComparer? EqualityComparer
         {
             get => _equalityComparer;
             set => _equalityComparer = value;
@@ -599,7 +596,7 @@ namespace Newtonsoft.Json
         /// The <see cref="JsonSerializer"/> will not use default settings 
         /// from <see cref="JsonConvert.DefaultSettings"/>.
         /// </returns>
-        public static JsonSerializer Create(JsonSerializerSettings settings)
+        public static JsonSerializer Create(JsonSerializerSettings? settings)
         {
             JsonSerializer serializer = Create();
 
@@ -624,7 +621,7 @@ namespace Newtonsoft.Json
         public static JsonSerializer CreateDefault()
         {
             // copy static to local variable to avoid concurrency issues
-            JsonSerializerSettings defaultSettings = JsonConvert.DefaultSettings?.Invoke();
+            JsonSerializerSettings? defaultSettings = JsonConvert.DefaultSettings?.Invoke();
 
             return Create(defaultSettings);
         }
@@ -640,7 +637,7 @@ namespace Newtonsoft.Json
         /// The <see cref="JsonSerializer"/> will use default settings 
         /// from <see cref="JsonConvert.DefaultSettings"/> as well as the specified <see cref="JsonSerializerSettings"/>.
         /// </returns>
-        public static JsonSerializer CreateDefault(JsonSerializerSettings settings)
+        public static JsonSerializer CreateDefault(JsonSerializerSettings? settings)
         {
             JsonSerializer serializer = CreateDefault();
             if (settings != null)
@@ -814,14 +811,14 @@ namespace Newtonsoft.Json
 
             SetupReader(
                 reader,
-                out CultureInfo previousCulture,
+                out CultureInfo? previousCulture,
                 out DateTimeZoneHandling? previousDateTimeZoneHandling,
                 out DateParseHandling? previousDateParseHandling,
                 out FloatParseHandling? previousFloatParseHandling,
                 out int? previousMaxDepth,
-                out string previousDateFormatString);
+                out string? previousDateFormatString);
 
-            TraceJsonReader traceJsonReader = (TraceWriter != null && TraceWriter.LevelFilter >= TraceLevel.Verbose)
+            TraceJsonReader? traceJsonReader = (TraceWriter != null && TraceWriter.LevelFilter >= TraceLevel.Verbose)
                 ? CreateTraceJsonReader(reader)
                 : null;
 
@@ -830,7 +827,7 @@ namespace Newtonsoft.Json
 
             if (traceJsonReader != null)
             {
-                TraceWriter.Trace(TraceLevel.Verbose, traceJsonReader.GetDeserializedJsonMessage(), null);
+                TraceWriter!.Trace(TraceLevel.Verbose, traceJsonReader.GetDeserializedJsonMessage(), null);
             }
 
             ResetReader(reader, previousCulture, previousDateTimeZoneHandling, previousDateParseHandling, previousFloatParseHandling, previousMaxDepth, previousDateFormatString);
@@ -842,7 +839,7 @@ namespace Newtonsoft.Json
         /// <param name="reader">The <see cref="JsonReader"/> that contains the JSON structure to deserialize.</param>
         /// <returns>The <see cref="Object"/> being deserialized.</returns>
         [DebuggerStepThrough]
-        public object Deserialize(JsonReader reader)
+        public object? Deserialize(JsonReader reader)
         {
             return Deserialize(reader, null);
         }
@@ -855,7 +852,7 @@ namespace Newtonsoft.Json
         /// <param name="objectType">The <see cref="Type"/> of object being deserialized.</param>
         /// <returns>The instance of <paramref name="objectType"/> being deserialized.</returns>
         [DebuggerStepThrough]
-        public object Deserialize(TextReader reader, Type objectType)
+        public object? Deserialize(TextReader reader, Type objectType)
         {
             return Deserialize(new JsonTextReader(reader), objectType);
         }
@@ -868,9 +865,12 @@ namespace Newtonsoft.Json
         /// <typeparam name="T">The type of the object to deserialize.</typeparam>
         /// <returns>The instance of <typeparamref name="T"/> being deserialized.</returns>
         [DebuggerStepThrough]
+        [return: MaybeNull]
         public T Deserialize<T>(JsonReader reader)
         {
+#pragma warning disable CS8601 // Possible null reference assignment.
             return (T)Deserialize(reader, typeof(T));
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -881,34 +881,34 @@ namespace Newtonsoft.Json
         /// <param name="objectType">The <see cref="Type"/> of object being deserialized.</param>
         /// <returns>The instance of <paramref name="objectType"/> being deserialized.</returns>
         [DebuggerStepThrough]
-        public object Deserialize(JsonReader reader, Type objectType)
+        public object? Deserialize(JsonReader reader, Type? objectType)
         {
             return DeserializeInternal(reader, objectType);
         }
 
-        internal virtual object DeserializeInternal(JsonReader reader, Type objectType)
+        internal virtual object? DeserializeInternal(JsonReader reader, Type? objectType)
         {
             ValidationUtils.ArgumentNotNull(reader, nameof(reader));
 
             SetupReader(
                 reader,
-                out CultureInfo previousCulture,
+                out CultureInfo? previousCulture,
                 out DateTimeZoneHandling? previousDateTimeZoneHandling,
                 out DateParseHandling? previousDateParseHandling,
                 out FloatParseHandling? previousFloatParseHandling,
                 out int? previousMaxDepth,
-                out string previousDateFormatString);
+                out string? previousDateFormatString);
 
-            TraceJsonReader traceJsonReader = (TraceWriter != null && TraceWriter.LevelFilter >= TraceLevel.Verbose)
+            TraceJsonReader? traceJsonReader = (TraceWriter != null && TraceWriter.LevelFilter >= TraceLevel.Verbose)
                 ? CreateTraceJsonReader(reader)
                 : null;
 
             JsonSerializerInternalReader serializerReader = new JsonSerializerInternalReader(this);
-            object value = serializerReader.Deserialize(traceJsonReader ?? reader, objectType, CheckAdditionalContent);
+            object? value = serializerReader.Deserialize(traceJsonReader ?? reader, objectType, CheckAdditionalContent);
 
             if (traceJsonReader != null)
             {
-                TraceWriter.Trace(TraceLevel.Verbose, traceJsonReader.GetDeserializedJsonMessage(), null);
+                TraceWriter!.Trace(TraceLevel.Verbose, traceJsonReader.GetDeserializedJsonMessage(), null);
             }
 
             ResetReader(reader, previousCulture, previousDateTimeZoneHandling, previousDateParseHandling, previousFloatParseHandling, previousMaxDepth, previousDateFormatString);
@@ -916,7 +916,7 @@ namespace Newtonsoft.Json
             return value;
         }
 
-        private void SetupReader(JsonReader reader, out CultureInfo previousCulture, out DateTimeZoneHandling? previousDateTimeZoneHandling, out DateParseHandling? previousDateParseHandling, out FloatParseHandling? previousFloatParseHandling, out int? previousMaxDepth, out string previousDateFormatString)
+        private void SetupReader(JsonReader reader, out CultureInfo? previousCulture, out DateTimeZoneHandling? previousDateTimeZoneHandling, out DateParseHandling? previousDateParseHandling, out FloatParseHandling? previousFloatParseHandling, out int? previousMaxDepth, out string? previousDateFormatString)
         {
             if (_culture != null && !_culture.Equals(reader.Culture))
             {
@@ -987,7 +987,7 @@ namespace Newtonsoft.Json
             }
         }
 
-        private void ResetReader(JsonReader reader, CultureInfo previousCulture, DateTimeZoneHandling? previousDateTimeZoneHandling, DateParseHandling? previousDateParseHandling, FloatParseHandling? previousFloatParseHandling, int? previousMaxDepth, string previousDateFormatString)
+        private void ResetReader(JsonReader reader, CultureInfo? previousCulture, DateTimeZoneHandling? previousDateTimeZoneHandling, DateParseHandling? previousDateParseHandling, FloatParseHandling? previousFloatParseHandling, int? previousMaxDepth, string? previousDateFormatString)
         {
             // reset reader back to previous options
             if (previousCulture != null)
@@ -1028,7 +1028,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="textWriter">The <see cref="TextWriter"/> used to write the JSON structure.</param>
         /// <param name="value">The <see cref="Object"/> to serialize.</param>
-        public void Serialize(TextWriter textWriter, object value)
+        public void Serialize(TextWriter textWriter, object? value)
         {
             Serialize(new JsonTextWriter(textWriter), value);
         }
@@ -1044,7 +1044,7 @@ namespace Newtonsoft.Json
         /// This parameter is used when <see cref="JsonSerializer.TypeNameHandling"/> is <see cref="Json.TypeNameHandling.Auto"/> to write out the type name if the type of the value does not match.
         /// Specifying the type is optional.
         /// </param>
-        public void Serialize(JsonWriter jsonWriter, object value, Type objectType)
+        public void Serialize(JsonWriter jsonWriter, object? value, Type? objectType)
         {
             SerializeInternal(jsonWriter, value, objectType);
         }
@@ -1060,7 +1060,7 @@ namespace Newtonsoft.Json
         /// This parameter is used when <see cref="TypeNameHandling"/> is Auto to write out the type name if the type of the value does not match.
         /// Specifying the type is optional.
         /// </param>
-        public void Serialize(TextWriter textWriter, object value, Type objectType)
+        public void Serialize(TextWriter textWriter, object? value, Type objectType)
         {
             Serialize(new JsonTextWriter(textWriter), value, objectType);
         }
@@ -1071,7 +1071,7 @@ namespace Newtonsoft.Json
         /// </summary>
         /// <param name="jsonWriter">The <see cref="JsonWriter"/> used to write the JSON structure.</param>
         /// <param name="value">The <see cref="Object"/> to serialize.</param>
-        public void Serialize(JsonWriter jsonWriter, object value)
+        public void Serialize(JsonWriter jsonWriter, object? value)
         {
             SerializeInternal(jsonWriter, value, null);
         }
@@ -1087,7 +1087,7 @@ namespace Newtonsoft.Json
             return traceReader;
         }
 
-        internal virtual void SerializeInternal(JsonWriter jsonWriter, object value, Type objectType)
+        internal virtual void SerializeInternal(JsonWriter jsonWriter, object? value, Type? objectType)
         {
             ValidationUtils.ArgumentNotNull(jsonWriter, nameof(jsonWriter));
 
@@ -1127,21 +1127,21 @@ namespace Newtonsoft.Json
                 jsonWriter.StringEscapeHandling = _stringEscapeHandling.GetValueOrDefault();
             }
 
-            CultureInfo previousCulture = null;
+            CultureInfo? previousCulture = null;
             if (_culture != null && !_culture.Equals(jsonWriter.Culture))
             {
                 previousCulture = jsonWriter.Culture;
                 jsonWriter.Culture = _culture;
             }
 
-            string previousDateFormatString = null;
+            string? previousDateFormatString = null;
             if (_dateFormatStringSet && jsonWriter.DateFormatString != _dateFormatString)
             {
                 previousDateFormatString = jsonWriter.DateFormatString;
                 jsonWriter.DateFormatString = _dateFormatString;
             }
 
-            TraceJsonWriter traceJsonWriter = (TraceWriter != null && TraceWriter.LevelFilter >= TraceLevel.Verbose)
+            TraceJsonWriter? traceJsonWriter = (TraceWriter != null && TraceWriter.LevelFilter >= TraceLevel.Verbose)
                 ? new TraceJsonWriter(jsonWriter)
                 : null;
 
@@ -1150,7 +1150,7 @@ namespace Newtonsoft.Json
 
             if (traceJsonWriter != null)
             {
-                TraceWriter.Trace(TraceLevel.Verbose, traceJsonWriter.GetSerializedJsonMessage(), null);
+                TraceWriter!.Trace(TraceLevel.Verbose, traceJsonWriter.GetSerializedJsonMessage(), null);
             }
 
             // reset writer back to previous options
@@ -1194,12 +1194,12 @@ namespace Newtonsoft.Json
             return _referenceResolver;
         }
 
-        internal JsonConverter GetMatchingConverter(Type type)
+        internal JsonConverter? GetMatchingConverter(Type type)
         {
             return GetMatchingConverter(_converters, type);
         }
 
-        internal static JsonConverter GetMatchingConverter(IList<JsonConverter> converters, Type objectType)
+        internal static JsonConverter? GetMatchingConverter(IList<JsonConverter>? converters, Type objectType)
         {
 #if DEBUG
             ValidationUtils.ArgumentNotNull(objectType, nameof(objectType));

Src/Newtonsoft.Json/JsonSerializerSettings.cs

@@ -69,11 +69,11 @@ namespace Newtonsoft.Json
         internal FloatFormatHandling? _floatFormatHandling;
         internal FloatParseHandling? _floatParseHandling;
         internal StringEscapeHandling? _stringEscapeHandling;
-        internal CultureInfo _culture;
+        internal CultureInfo? _culture;
         internal bool? _checkAdditionalContent;
         internal int? _maxDepth;
         internal bool _maxDepthSet;
-        internal string _dateFormatString;
+        internal string? _dateFormatString;
         internal bool _dateFormatStringSet;
         internal TypeNameAssemblyFormatHandling? _typeNameAssemblyFormatHandling;
         internal DefaultValueHandling? _defaultValueHandling;
@@ -225,27 +225,27 @@ namespace Newtonsoft.Json
         /// serializing .NET objects to JSON and vice versa.
         /// </summary>
         /// <value>The contract resolver.</value>
-        public IContractResolver ContractResolver { get; set; }
+        public IContractResolver? ContractResolver { get; set; }
 
         /// <summary>
         /// Gets or sets the equality comparer used by the serializer when comparing references.
         /// </summary>
         /// <value>The equality comparer.</value>
-        public IEqualityComparer EqualityComparer { get; set; }
+        public IEqualityComparer? EqualityComparer { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="IReferenceResolver"/> used by the serializer when resolving references.
         /// </summary>
         /// <value>The reference resolver.</value>
         [Obsolete("ReferenceResolver property is obsolete. Use the ReferenceResolverProvider property to set the IReferenceResolver: settings.ReferenceResolverProvider = () => resolver")]
-        public IReferenceResolver ReferenceResolver
+        public IReferenceResolver? ReferenceResolver
         {
             get => ReferenceResolverProvider?.Invoke();
             set
             {
                 ReferenceResolverProvider = (value != null)
                     ? () => value
-                    : (Func<IReferenceResolver>)null;
+                    : (Func<IReferenceResolver?>?)null;
             }
         }
 
@@ -253,20 +253,20 @@ namespace Newtonsoft.Json
         /// Gets or sets a function that creates the <see cref="IReferenceResolver"/> used by the serializer when resolving references.
         /// </summary>
         /// <value>A function that creates the <see cref="IReferenceResolver"/> used by the serializer when resolving references.</value>
-        public Func<IReferenceResolver> ReferenceResolverProvider { get; set; }
+        public Func<IReferenceResolver?>? ReferenceResolverProvider { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="ITraceWriter"/> used by the serializer when writing trace messages.
         /// </summary>
         /// <value>The trace writer.</value>
-        public ITraceWriter TraceWriter { get; set; }
+        public ITraceWriter? TraceWriter { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="SerializationBinder"/> used by the serializer when resolving type names.
         /// </summary>
         /// <value>The binder.</value>
         [Obsolete("Binder is obsolete. Use SerializationBinder instead.")]
-        public SerializationBinder Binder
+        public SerializationBinder? Binder
         {
             get
             {
@@ -289,13 +289,13 @@ namespace Newtonsoft.Json
         /// Gets or sets the <see cref="ISerializationBinder"/> used by the serializer when resolving type names.
         /// </summary>
         /// <value>The binder.</value>
-        public ISerializationBinder SerializationBinder { get; set; }
+        public ISerializationBinder? SerializationBinder { get; set; }
 
         /// <summary>
         /// Gets or sets the error handler called during serialization and deserialization.
         /// </summary>
         /// <value>The error handler called during serialization and deserialization.</value>
-        public EventHandler<ErrorEventArgs> Error { get; set; }
+        public EventHandler<ErrorEventArgs>? Error { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="StreamingContext"/> used by the serializer when invoking serialization callback methods.

Src/Newtonsoft.Json/JsonTextReader.Async.cs

@@ -34,6 +34,7 @@ using System.Numerics;
 using System.Threading.Tasks;
 using Newtonsoft.Json.Serialization;
 using Newtonsoft.Json.Utilities;
+using System.Diagnostics;
 
 namespace Newtonsoft.Json
 {
@@ -110,6 +111,8 @@ namespace Newtonsoft.Json
 
         private async Task<bool> ParsePostValueAsync(bool ignoreComments, CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -193,6 +196,8 @@ namespace Newtonsoft.Json
 
         private async Task<bool> ReadFromFinishedAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             if (await EnsureCharsAsync(0, false, cancellationToken).ConfigureAwait(false))
             {
                 await EatWhitespaceAsync(cancellationToken).ConfigureAwait(false);
@@ -222,6 +227,8 @@ namespace Newtonsoft.Json
 
         private async Task<int> ReadDataAsync(bool append, int charsRequired, CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             if (_isEndOfFile)
             {
                 return 0;
@@ -244,6 +251,8 @@ namespace Newtonsoft.Json
 
         private async Task<bool> ParseValueAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -373,6 +382,8 @@ namespace Newtonsoft.Json
 
         private async Task ReadStringIntoBufferAsync(char quote, CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             int charPos = _charPos;
             int initialPosition = _charPos;
             int lastWritePosition = _charPos;
@@ -589,6 +600,8 @@ namespace Newtonsoft.Json
 
         private async Task<bool> ParseObjectAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -646,6 +659,8 @@ namespace Newtonsoft.Json
 
         private async Task ParseCommentAsync(bool setToken, CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             // should have already parsed / character before reaching this method
             _charPos++;
 
@@ -742,6 +757,8 @@ namespace Newtonsoft.Json
 
         private async Task EatWhitespaceAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -798,6 +815,8 @@ namespace Newtonsoft.Json
 
         private async Task<bool> MatchValueWithTrailingSeparatorAsync(string value, CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             // will match value and then move to the next character, checking that it is a separator character
             if (!await MatchValueAsync(value, cancellationToken).ConfigureAwait(false))
             {
@@ -812,7 +831,7 @@ namespace Newtonsoft.Json
             return IsSeparator(_chars[_charPos]) || _chars[_charPos] == '\0';
         }
 
-        private async Task MatchAndSetAsync(string value, JsonToken newToken, object tokenValue, CancellationToken cancellationToken)
+        private async Task MatchAndSetAsync(string value, JsonToken newToken, object? tokenValue, CancellationToken cancellationToken)
         {
             if (await MatchValueWithTrailingSeparatorAsync(value, cancellationToken).ConfigureAwait(false))
             {
@@ -841,6 +860,8 @@ namespace Newtonsoft.Json
 
         private async Task ParseConstructorAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             if (await MatchValueWithTrailingSeparatorAsync("new", cancellationToken).ConfigureAwait(false))
             {
                 await EatWhitespaceAsync(cancellationToken).ConfigureAwait(false);
@@ -939,6 +960,8 @@ namespace Newtonsoft.Json
 
         private async Task ParseNumberAsync(ReadType readType, CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             ShiftBufferIfNeeded();
 
             char firstChar = _chars[_charPos];
@@ -956,6 +979,8 @@ namespace Newtonsoft.Json
 
         private async Task<bool> ParsePropertyAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             char firstChar = _chars[_charPos];
             char quoteChar;
 
@@ -1008,6 +1033,8 @@ namespace Newtonsoft.Json
 
         private async Task ReadNumberIntoBufferAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             int charPos = _charPos;
 
             while (true)
@@ -1042,6 +1069,8 @@ namespace Newtonsoft.Json
 
         private async Task ParseUnquotedPropertyAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             int initialPosition = _charPos;
 
             // parse unquoted property name until whitespace or colon
@@ -1091,6 +1120,8 @@ namespace Newtonsoft.Json
 
         private async Task HandleNullAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             if (await EnsureCharsAsync(1, true, cancellationToken).ConfigureAwait(false))
             {
                 if (_chars[_charPos + 1] == 'u')
@@ -1109,6 +1140,8 @@ namespace Newtonsoft.Json
 
         private async Task ReadFinishedAsync(CancellationToken cancellationToken)
         {
+            Debug.Assert(_chars != null);
+
             if (await EnsureCharsAsync(0, false, cancellationToken).ConfigureAwait(false))
             {
                 await EatWhitespaceAsync(cancellationToken).ConfigureAwait(false);
@@ -1131,9 +1164,10 @@ namespace Newtonsoft.Json
             SetToken(JsonToken.None);
         }
 
-        private async Task<object> ReadStringValueAsync(ReadType readType, CancellationToken cancellationToken)
+        private async Task<object?> ReadStringValueAsync(ReadType readType, CancellationToken cancellationToken)
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             switch (_currentState)
             {
@@ -1266,9 +1300,10 @@ namespace Newtonsoft.Json
             }
         }
 
-        private async Task<object> ReadNumberValueAsync(ReadType readType, CancellationToken cancellationToken)
+        private async Task<object?> ReadNumberValueAsync(ReadType readType, CancellationToken cancellationToken)
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             switch (_currentState)
             {
@@ -1395,6 +1430,7 @@ namespace Newtonsoft.Json
         internal async Task<bool?> DoReadAsBooleanAsync(CancellationToken cancellationToken)
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             switch (_currentState)
             {
@@ -1522,14 +1558,16 @@ namespace Newtonsoft.Json
         /// property returns the <see cref="byte"/>[]. This result will be <c>null</c> at the end of an array.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task<byte[]> ReadAsBytesAsync(CancellationToken cancellationToken = default)
+        public override Task<byte[]?> ReadAsBytesAsync(CancellationToken cancellationToken = default)
         {
             return _safeAsync ? DoReadAsBytesAsync(cancellationToken) : base.ReadAsBytesAsync(cancellationToken);
         }
 
-        internal async Task<byte[]> DoReadAsBytesAsync(CancellationToken cancellationToken)
+        internal async Task<byte[]?> DoReadAsBytesAsync(CancellationToken cancellationToken)
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
+
             bool isWrapped = false;
 
             switch (_currentState)
@@ -1563,7 +1601,7 @@ namespace Newtonsoft.Json
                             case '"':
                             case '\'':
                                 await ParseStringAsync(currentChar, ReadType.ReadAsBytes, cancellationToken).ConfigureAwait(false);
-                                byte[] data = (byte[])Value;
+                                byte[]? data = (byte[]?)Value;
                                 if (isWrapped)
                                 {
                                     await ReaderReadAndAssertAsync(cancellationToken).ConfigureAwait(false);
@@ -1753,14 +1791,14 @@ namespace Newtonsoft.Json
         /// property returns the <see cref="string"/>. This result will be <c>null</c> at the end of an array.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task<string> ReadAsStringAsync(CancellationToken cancellationToken = default)
+        public override Task<string?> ReadAsStringAsync(CancellationToken cancellationToken = default)
         {
             return _safeAsync ? DoReadAsStringAsync(cancellationToken) : base.ReadAsStringAsync(cancellationToken);
         }
 
-        internal async Task<string> DoReadAsStringAsync(CancellationToken cancellationToken)
+        internal async Task<string?> DoReadAsStringAsync(CancellationToken cancellationToken)
         {
-            return (string)await ReadStringValueAsync(ReadType.ReadAsString, cancellationToken).ConfigureAwait(false);
+            return (string?)await ReadStringValueAsync(ReadType.ReadAsString, cancellationToken).ConfigureAwait(false);
         }
     }
 }

Src/Newtonsoft.Json/JsonTextReader.cs

@@ -27,6 +27,7 @@ using System;
 using System.Runtime.CompilerServices;
 using System.IO;
 using System.Globalization;
+using System.Diagnostics;
 #if HAVE_BIG_INTEGER
 using System.Numerics;
 #endif
@@ -66,7 +67,7 @@ namespace Newtonsoft.Json
 #endif
 
         private readonly TextReader _reader;
-        private char[] _chars;
+        private char[]? _chars;
         private int _charsUsed;
         private int _charPos;
         private int _lineStartPos;
@@ -74,7 +75,7 @@ namespace Newtonsoft.Json
         private bool _isEndOfFile;
         private StringBuffer _stringBuffer;
         private StringReference _stringReference;
-        private IArrayPool<char> _arrayPool;
+        private IArrayPool<char>? _arrayPool;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonTextReader"/> class with the specified <see cref="TextReader"/>.
@@ -96,7 +97,7 @@ namespace Newtonsoft.Json
         }
 
 #if DEBUG
-        internal char[] CharBuffer
+        internal char[]? CharBuffer
         {
             get => _chars;
             set => _chars = value;
@@ -108,12 +109,12 @@ namespace Newtonsoft.Json
         /// <summary>
         /// Gets or sets the reader's property name table.
         /// </summary>
-        public JsonNameTable PropertyNameTable { get; set; }
+        public JsonNameTable? PropertyNameTable { get; set; }
 
         /// <summary>
         /// Gets or sets the reader's character buffer pool.
         /// </summary>
-        public IArrayPool<char> ArrayPool
+        public IArrayPool<char>? ArrayPool
         {
             get => _arrayPool;
             set
@@ -137,6 +138,8 @@ namespace Newtonsoft.Json
 
         private void SetNewLine(bool hasNextChar)
         {
+            Debug.Assert(_chars != null);
+
             if (hasNextChar && _chars[_charPos] == StringUtils.LineFeed)
             {
                 _charPos++;
@@ -249,6 +252,8 @@ namespace Newtonsoft.Json
 
         private void ShiftBufferIfNeeded()
         {
+            Debug.Assert(_chars != null);
+
             // once in the last 10% of the buffer, or buffer is already very large then
             // shift the remaining content to the start to avoid unnecessarily increasing
             // the buffer size when reading numbers/strings
@@ -275,6 +280,8 @@ namespace Newtonsoft.Json
 
         private void PrepareBufferForReadData(bool append, int charsRequired)
         {
+            Debug.Assert(_chars != null);
+
             // char buffer is full
             if (_charsUsed + charsRequired >= _chars.Length - 1)
             {
@@ -338,6 +345,7 @@ namespace Newtonsoft.Json
             }
 
             PrepareBufferForReadData(append, charsRequired);
+            Debug.Assert(_chars != null);
 
             int attemptCharReadCount = _chars.Length - _charsUsed - 1;
 
@@ -406,6 +414,7 @@ namespace Newtonsoft.Json
         public override bool Read()
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             while (true)
             {
@@ -476,18 +485,20 @@ namespace Newtonsoft.Json
         /// Reads the next JSON token from the underlying <see cref="TextReader"/> as a <see cref="String"/>.
         /// </summary>
         /// <returns>A <see cref="String"/>. This method will return <c>null</c> at the end of an array.</returns>
-        public override string ReadAsString()
+        public override string? ReadAsString()
         {
-            return (string)ReadStringValue(ReadType.ReadAsString);
+            return (string?)ReadStringValue(ReadType.ReadAsString);
         }
 
         /// <summary>
         /// Reads the next JSON token from the underlying <see cref="TextReader"/> as a <see cref="Byte"/>[].
         /// </summary>
         /// <returns>A <see cref="Byte"/>[] or <c>null</c> if the next JSON token is null. This method will return <c>null</c> at the end of an array.</returns>
-        public override byte[] ReadAsBytes()
+        public override byte[]? ReadAsBytes()
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
+
             bool isWrapped = false;
 
             switch (_currentState)
@@ -520,7 +531,7 @@ namespace Newtonsoft.Json
                             case '"':
                             case '\'':
                                 ParseString(currentChar, ReadType.ReadAsBytes);
-                                byte[] data = (byte[])Value;
+                                byte[]? data = (byte[]?)Value;
                                 if (isWrapped)
                                 {
                                     ReaderReadAndAssert();
@@ -589,9 +600,10 @@ namespace Newtonsoft.Json
             }
         }
 
-        private object ReadStringValue(ReadType readType)
+        private object? ReadStringValue(ReadType readType)
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             switch (_currentState)
             {
@@ -718,7 +730,7 @@ namespace Newtonsoft.Json
             }
         }
 
-        private object FinishReadQuotedStringValue(ReadType readType)
+        private object? FinishReadQuotedStringValue(ReadType readType)
         {
             switch (readType)
             {
@@ -731,7 +743,7 @@ namespace Newtonsoft.Json
                         return time;
                     }
 
-                    return ReadDateTimeString((string)Value);
+                    return ReadDateTimeString((string?)Value);
 #if HAVE_DATE_TIME_OFFSET
                 case ReadType.ReadAsDateTimeOffset:
                     if (Value is DateTimeOffset offset)
@@ -739,7 +751,7 @@ namespace Newtonsoft.Json
                         return offset;
                     }
 
-                    return ReadDateTimeOffsetString((string)Value);
+                    return ReadDateTimeOffsetString((string?)Value);
 #endif
                 default:
                     throw new ArgumentOutOfRangeException(nameof(readType));
@@ -758,6 +770,7 @@ namespace Newtonsoft.Json
         public override bool? ReadAsBoolean()
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             switch (_currentState)
             {
@@ -892,9 +905,10 @@ namespace Newtonsoft.Json
             SetStateBasedOnCurrent();
         }
 
-        private object ReadNumberValue(ReadType readType)
+        private object? ReadNumberValue(ReadType readType)
         {
             EnsureBuffer();
+            Debug.Assert(_chars != null);
 
             switch (_currentState)
             {
@@ -1002,7 +1016,7 @@ namespace Newtonsoft.Json
             }
         }
 
-        private object FinishReadQuotedNumber(ReadType readType)
+        private object? FinishReadQuotedNumber(ReadType readType)
         {
             switch (readType)
             {
@@ -1048,6 +1062,8 @@ namespace Newtonsoft.Json
 
         private void HandleNull()
         {
+            Debug.Assert(_chars != null);
+
             if (EnsureChars(1, true))
             {
                 char next = _chars[_charPos + 1];
@@ -1068,6 +1084,8 @@ namespace Newtonsoft.Json
 
         private void ReadFinished()
         {
+            Debug.Assert(_chars != null);
+
             if (EnsureChars(0, false))
             {
                 EatWhitespace();
@@ -1117,6 +1135,8 @@ namespace Newtonsoft.Json
 
         private void ReadStringIntoBuffer(char quote)
         {
+            Debug.Assert(_chars != null);
+
             int charPos = _charPos;
             int initialPosition = _charPos;
             int lastWritePosition = _charPos;
@@ -1270,6 +1290,8 @@ namespace Newtonsoft.Json
 
         private void FinishReadStringIntoBuffer(int charPos, int initialPosition, int lastWritePosition)
         {
+            Debug.Assert(_chars != null);
+
             if (initialPosition == lastWritePosition)
             {
                 _stringReference = new StringReference(_chars, initialPosition, charPos - initialPosition);
@@ -1283,7 +1305,7 @@ namespace Newtonsoft.Json
                     _stringBuffer.Append(_arrayPool, _chars, lastWritePosition, charPos - lastWritePosition);
                 }
 
-                _stringReference = new StringReference(_stringBuffer.InternalBuffer, 0, _stringBuffer.Position);
+                _stringReference = new StringReference(_stringBuffer.InternalBuffer!, 0, _stringBuffer.Position);
             }
 
             _charPos = charPos + 1;
@@ -1291,6 +1313,8 @@ namespace Newtonsoft.Json
 
         private void WriteCharToBuffer(char writeChar, int lastWritePosition, int writeToPosition)
         {
+            Debug.Assert(_chars != null);
+
             if (writeToPosition > lastWritePosition)
             {
                 _stringBuffer.Append(_arrayPool, _chars, lastWritePosition, writeToPosition - lastWritePosition);
@@ -1301,6 +1325,8 @@ namespace Newtonsoft.Json
 
         private char ConvertUnicode(bool enoughChars)
         {
+            Debug.Assert(_chars != null);
+
             if (enoughChars)
             {
                 if (ConvertUtils.TryHexTextToInt(_chars, _charPos, _charPos + 4, out int value))
@@ -1327,6 +1353,8 @@ namespace Newtonsoft.Json
 
         private void ReadNumberIntoBuffer()
         {
+            Debug.Assert(_chars != null);
+
             int charPos = _charPos;
 
             while (true)
@@ -1411,6 +1439,8 @@ namespace Newtonsoft.Json
 
         private bool ParsePostValue(bool ignoreComments)
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -1491,6 +1521,8 @@ namespace Newtonsoft.Json
 
         private bool ParseObject()
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -1545,6 +1577,8 @@ namespace Newtonsoft.Json
 
         private bool ParseProperty()
         {
+            Debug.Assert(_chars != null);
+
             char firstChar = _chars[_charPos];
             char quoteChar;
 
@@ -1566,7 +1600,7 @@ namespace Newtonsoft.Json
                 throw JsonReaderException.Create(this, "Invalid property identifier character: {0}.".FormatWith(CultureInfo.InvariantCulture, _chars[_charPos]));
             }
 
-            string propertyName;
+            string? propertyName;
 
             if (PropertyNameTable != null)
             {
@@ -1606,6 +1640,8 @@ namespace Newtonsoft.Json
 
         private void ParseUnquotedProperty()
         {
+            Debug.Assert(_chars != null);
+
             int initialPosition = _charPos;
 
             // parse unquoted property name until whitespace or colon
@@ -1637,6 +1673,8 @@ namespace Newtonsoft.Json
 
         private bool ReadUnquotedPropertyReportIfDone(char currentChar, int initialPosition)
         {
+            Debug.Assert(_chars != null);
+
             if (ValidIdentifierChar(currentChar))
             {
                 _charPos++;
@@ -1654,6 +1692,8 @@ namespace Newtonsoft.Json
 
         private bool ParseValue()
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -1794,6 +1834,8 @@ namespace Newtonsoft.Json
 
         private void EatWhitespace()
         {
+            Debug.Assert(_chars != null);
+
             while (true)
             {
                 char currentChar = _chars[_charPos];
@@ -1835,6 +1877,8 @@ namespace Newtonsoft.Json
 
         private void ParseConstructor()
         {
+            Debug.Assert(_chars != null);
+
             if (MatchValueWithTrailingSeparator("new"))
             {
                 EatWhitespace();
@@ -1919,6 +1963,7 @@ namespace Newtonsoft.Json
         private void ParseNumber(ReadType readType)
         {
             ShiftBufferIfNeeded();
+            Debug.Assert(_chars != null);
 
             char firstChar = _chars[_charPos];
             int initialPosition = _charPos;
@@ -1929,7 +1974,9 @@ namespace Newtonsoft.Json
         }
 
         private void ParseReadNumber(ReadType readType, char firstChar, int initialPosition)
-        { 
+        {
+            Debug.Assert(_chars != null);
+
             // set state to PostValue now so that if there is an error parsing the number then the reader can continue
             SetPostValueState(true);
 
@@ -2190,7 +2237,7 @@ namespace Newtonsoft.Json
             SetToken(numberType, numberValue, false);
         }
 
-        private JsonReaderException ThrowReaderError(string message, Exception ex = null)
+        private JsonReaderException ThrowReaderError(string message, Exception? ex = null)
         {
             SetToken(JsonToken.Undefined, null, false);
             return JsonReaderException.Create(this, message, ex);
@@ -2210,6 +2257,8 @@ namespace Newtonsoft.Json
 
         private void ParseComment(bool setToken)
         {
+            Debug.Assert(_chars != null);
+
             // should have already parsed / character before reaching this method
             _charPos++;
 
@@ -2315,6 +2364,8 @@ namespace Newtonsoft.Json
 
         private bool MatchValue(bool enoughChars, string value)
         {
+            Debug.Assert(_chars != null);
+
             if (!enoughChars)
             {
                 _charPos = _charsUsed;
@@ -2337,6 +2388,8 @@ namespace Newtonsoft.Json
 
         private bool MatchValueWithTrailingSeparator(string value)
         {
+            Debug.Assert(_chars != null);
+
             // will match value and then move to the next character, checking that it is a separator character
             bool match = MatchValue(value);
 
@@ -2355,6 +2408,8 @@ namespace Newtonsoft.Json
 
         private bool IsSeparator(char c)
         {
+            Debug.Assert(_chars != null);
+
             switch (c)
             {
                 case '}':

Src/Newtonsoft.Json/JsonTextWriter.Async.cs

@@ -33,6 +33,7 @@ using System.Numerics;
 #endif
 using System.Threading.Tasks;
 using Newtonsoft.Json.Utilities;
+using System.Diagnostics;
 
 namespace Newtonsoft.Json
 {
@@ -164,6 +165,7 @@ namespace Newtonsoft.Json
             int currentIndentCount = Top * _indentation;
 
             int newLineLen = SetIndentChars();
+            Debug.Assert(_indentChars != null);
 
             if (currentIndentCount <= IndentCharBufferSize)
             {
@@ -175,6 +177,8 @@ namespace Newtonsoft.Json
 
         private async Task WriteIndentAsync(int currentIndentCount, int newLineLen, CancellationToken cancellationToken)
         {
+            Debug.Assert(_indentChars != null);
+
             await _writer.WriteAsync(_indentChars, 0, newLineLen + Math.Min(currentIndentCount, IndentCharBufferSize), cancellationToken).ConfigureAwait(false);
 
             while ((currentIndentCount -= IndentCharBufferSize) > 0)
@@ -225,12 +229,12 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteRawAsync(string json, CancellationToken cancellationToken = default)
+        public override Task WriteRawAsync(string? json, CancellationToken cancellationToken = default)
         {
             return _safeAsync ? DoWriteRawAsync(json, cancellationToken) : base.WriteRawAsync(json, cancellationToken);
         }
 
-        internal Task DoWriteRawAsync(string json, CancellationToken cancellationToken)
+        internal Task DoWriteRawAsync(string? json, CancellationToken cancellationToken)
         {
             return _writer.WriteAsync(json, cancellationToken);
         }
@@ -260,7 +264,7 @@ namespace Newtonsoft.Json
             }
 
             int length = WriteNumberToBuffer(uvalue, negative);
-            return _writer.WriteAsync(_writeBuffer, 0, length, cancellationToken);
+            return _writer.WriteAsync(_writeBuffer!, 0, length, cancellationToken);
         }
 
         private Task WriteIntegerValueAsync(ulong uvalue, bool negative, CancellationToken cancellationToken)
@@ -298,7 +302,7 @@ namespace Newtonsoft.Json
 
         private Task WriteEscapedStringAsync(string value, bool quote, CancellationToken cancellationToken)
         {
-            return JavaScriptUtils.WriteEscapedJavaScriptStringAsync(_writer, value, _quoteChar, quote, _charEscapeFlags, StringEscapeHandling, this, _writeBuffer, cancellationToken);
+            return JavaScriptUtils.WriteEscapedJavaScriptStringAsync(_writer, value, _quoteChar, quote, _charEscapeFlags!, StringEscapeHandling, this, _writeBuffer!, cancellationToken);
         }
 
         /// <summary>
@@ -585,7 +589,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteValueAsync(byte[] value, CancellationToken cancellationToken = default)
+        public override Task WriteValueAsync(byte[]? value, CancellationToken cancellationToken = default)
         {
             return _safeAsync ? (value == null ? WriteNullAsync(cancellationToken) : WriteValueNonNullAsync(value, cancellationToken)) : base.WriteValueAsync(value, cancellationToken);
         }
@@ -657,7 +661,7 @@ namespace Newtonsoft.Json
             {
                 int length = WriteValueToBuffer(value);
 
-                await _writer.WriteAsync(_writeBuffer, 0, length, cancellationToken).ConfigureAwait(false);
+                await _writer.WriteAsync(_writeBuffer!, 0, length, cancellationToken).ConfigureAwait(false);
             }
             else
             {
@@ -706,7 +710,7 @@ namespace Newtonsoft.Json
             {
                 int length = WriteValueToBuffer(value);
 
-                await _writer.WriteAsync(_writeBuffer, 0, length, cancellationToken).ConfigureAwait(false);
+                await _writer.WriteAsync(_writeBuffer!, 0, length, cancellationToken).ConfigureAwait(false);
             }
             else
             {
@@ -953,7 +957,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteValueAsync(object value, CancellationToken cancellationToken = default)
+        public override Task WriteValueAsync(object? value, CancellationToken cancellationToken = default)
         {
             if (_safeAsync)
             {
@@ -1046,12 +1050,12 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteValueAsync(string value, CancellationToken cancellationToken = default)
+        public override Task WriteValueAsync(string? value, CancellationToken cancellationToken = default)
         {
             return _safeAsync ? DoWriteValueAsync(value, cancellationToken) : base.WriteValueAsync(value, cancellationToken);
         }
 
-        internal Task DoWriteValueAsync(string value, CancellationToken cancellationToken)
+        internal Task DoWriteValueAsync(string? value, CancellationToken cancellationToken)
         {
             Task task = InternalWriteValueAsync(JsonToken.String, cancellationToken);
             if (task.IsCompletedSucessfully())
@@ -1062,7 +1066,7 @@ namespace Newtonsoft.Json
             return DoWriteValueAsync(task, value, cancellationToken);
         }
 
-        private async Task DoWriteValueAsync(Task task, string value, CancellationToken cancellationToken)
+        private async Task DoWriteValueAsync(Task task, string? value, CancellationToken cancellationToken)
         {
             await task.ConfigureAwait(false);
             await (value == null ? _writer.WriteAsync(JsonConvert.Null, cancellationToken) : WriteEscapedStringAsync(value, true, cancellationToken)).ConfigureAwait(false);
@@ -1181,7 +1185,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteValueAsync(Uri value, CancellationToken cancellationToken = default)
+        public override Task WriteValueAsync(Uri? value, CancellationToken cancellationToken = default)
         {
             return _safeAsync ? (value == null ? WriteNullAsync(cancellationToken) : WriteValueNotNullAsync(value, cancellationToken)) : base.WriteValueAsync(value, cancellationToken);
         }
@@ -1244,16 +1248,16 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteCommentAsync(string text, CancellationToken cancellationToken = default)
+        public override Task WriteCommentAsync(string? text, CancellationToken cancellationToken = default)
         {
             return _safeAsync ? DoWriteCommentAsync(text, cancellationToken) : base.WriteCommentAsync(text, cancellationToken);
         }
 
-        internal async Task DoWriteCommentAsync(string text, CancellationToken cancellationToken)
+        internal async Task DoWriteCommentAsync(string? text, CancellationToken cancellationToken)
         {
             await InternalWriteCommentAsync(cancellationToken).ConfigureAwait(false);
             await _writer.WriteAsync("/*", cancellationToken).ConfigureAwait(false);
-            await _writer.WriteAsync(text, cancellationToken).ConfigureAwait(false);
+            await _writer.WriteAsync(text ?? string.Empty, cancellationToken).ConfigureAwait(false);
             await _writer.WriteAsync("*/", cancellationToken).ConfigureAwait(false);
         }
 
@@ -1301,12 +1305,12 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>Derived classes must override this method to get asynchronous behaviour. Otherwise it will
         /// execute synchronously, returning an already-completed task.</remarks>
-        public override Task WriteRawValueAsync(string json, CancellationToken cancellationToken = default)
+        public override Task WriteRawValueAsync(string? json, CancellationToken cancellationToken = default)
         {
             return _safeAsync ? DoWriteRawValueAsync(json, cancellationToken) : base.WriteRawValueAsync(json, cancellationToken);
         }
 
-        internal Task DoWriteRawValueAsync(string json, CancellationToken cancellationToken)
+        internal Task DoWriteRawValueAsync(string? json, CancellationToken cancellationToken)
         {
             UpdateScopeWithFinishedValue();
             Task task = AutoCompleteAsync(JsonToken.Undefined, cancellationToken);
@@ -1318,7 +1322,7 @@ namespace Newtonsoft.Json
             return DoWriteRawValueAsync(task, json, cancellationToken);
         }
 
-        private async Task DoWriteRawValueAsync(Task task, string json, CancellationToken cancellationToken)
+        private async Task DoWriteRawValueAsync(Task task, string? json, CancellationToken cancellationToken)
         {
             await task.ConfigureAwait(false);
             await WriteRawAsync(json, cancellationToken).ConfigureAwait(false);
@@ -1331,7 +1335,7 @@ namespace Newtonsoft.Json
                 length = 35;
             }
 
-            char[] buffer = _writeBuffer;
+            char[]? buffer = _writeBuffer;
             if (buffer == null)
             {
                 return _writeBuffer = BufferUtils.RentBuffer(_arrayPool, length);

Src/Newtonsoft.Json/JsonTextWriter.cs

@@ -33,6 +33,7 @@ using System.Text;
 using System.IO;
 using System.Xml;
 using Newtonsoft.Json.Utilities;
+using System.Diagnostics;
 
 namespace Newtonsoft.Json
 {
@@ -43,15 +44,15 @@ namespace Newtonsoft.Json
     {
         private const int IndentCharBufferSize = 12;
         private readonly TextWriter _writer;
-        private Base64Encoder _base64Encoder;
+        private Base64Encoder? _base64Encoder;
         private char _indentChar;
         private int _indentation;
         private char _quoteChar;
         private bool _quoteName;
-        private bool[] _charEscapeFlags;
-        private char[] _writeBuffer;
-        private IArrayPool<char> _arrayPool;
-        private char[] _indentChars;
+        private bool[]? _charEscapeFlags;
+        private char[]? _writeBuffer;
+        private IArrayPool<char>? _arrayPool;
+        private char[]? _indentChars;
 
         private Base64Encoder Base64Encoder
         {
@@ -69,7 +70,7 @@ namespace Newtonsoft.Json
         /// <summary>
         /// Gets or sets the writer's character array pool.
         /// </summary>
-        public IArrayPool<char> ArrayPool
+        public IArrayPool<char>? ArrayPool
         {
             get => _arrayPool;
             set
@@ -342,7 +343,7 @@ namespace Newtonsoft.Json
             {
                 for (int i = 0; i != newLineLen; ++i)
                 {
-                    if (writerNewLine[i] != _indentChars[i])
+                    if (writerNewLine[i] != _indentChars![i])
                     {
                         match = false;
                         break;
@@ -387,7 +388,7 @@ namespace Newtonsoft.Json
         /// An error will raised if the value cannot be written as a single JSON token.
         /// </summary>
         /// <param name="value">The <see cref="Object"/> value to write.</param>
-        public override void WriteValue(object value)
+        public override void WriteValue(object? value)
         {
 #if HAVE_BIG_INTEGER
             if (value is BigInteger i)
@@ -424,7 +425,7 @@ namespace Newtonsoft.Json
         /// Writes raw JSON.
         /// </summary>
         /// <param name="json">The raw JSON to write.</param>
-        public override void WriteRaw(string json)
+        public override void WriteRaw(string? json)
         {
             InternalWriteRaw();
 
@@ -435,7 +436,7 @@ namespace Newtonsoft.Json
         /// Writes a <see cref="String"/> value.
         /// </summary>
         /// <param name="value">The <see cref="String"/> value to write.</param>
-        public override void WriteValue(string value)
+        public override void WriteValue(string? value)
         {
             InternalWriteValue(JsonToken.String);
 
@@ -452,7 +453,7 @@ namespace Newtonsoft.Json
         private void WriteEscapedString(string value, bool quote)
         {
             EnsureWriteBuffer();
-            JavaScriptUtils.WriteEscapedJavaScriptString(_writer, value, _quoteChar, quote, _charEscapeFlags, StringEscapeHandling, _arrayPool, ref _writeBuffer);
+            JavaScriptUtils.WriteEscapedJavaScriptString(_writer, value, _quoteChar, quote, _charEscapeFlags!, StringEscapeHandling, _arrayPool, ref _writeBuffer);
         }
 
         /// <summary>
@@ -649,6 +650,7 @@ namespace Newtonsoft.Json
         private int WriteValueToBuffer(DateTime value)
         {
             EnsureWriteBuffer();
+            Debug.Assert(_writeBuffer != null);
 
             int pos = 0;
             _writeBuffer[pos++] = _quoteChar;
@@ -661,7 +663,7 @@ namespace Newtonsoft.Json
         /// Writes a <see cref="Byte"/>[] value.
         /// </summary>
         /// <param name="value">The <see cref="Byte"/>[] value to write.</param>
-        public override void WriteValue(byte[] value)
+        public override void WriteValue(byte[]? value)
         {
             if (value == null)
             {
@@ -703,6 +705,7 @@ namespace Newtonsoft.Json
         private int WriteValueToBuffer(DateTimeOffset value)
         {
             EnsureWriteBuffer();
+            Debug.Assert(_writeBuffer != null);
 
             int pos = 0;
             _writeBuffer[pos++] = _quoteChar;
@@ -720,7 +723,7 @@ namespace Newtonsoft.Json
         {
             InternalWriteValue(JsonToken.String);
 
-            string text = null;
+            string text;
 
 #if HAVE_CHAR_TO_STRING_WITH_CULTURE
             text = value.ToString("D", CultureInfo.InvariantCulture);
@@ -757,7 +760,7 @@ namespace Newtonsoft.Json
         /// Writes a <see cref="Uri"/> value.
         /// </summary>
         /// <param name="value">The <see cref="Uri"/> value to write.</param>
-        public override void WriteValue(Uri value)
+        public override void WriteValue(Uri? value)
         {
             if (value == null)
             {
@@ -775,7 +778,7 @@ namespace Newtonsoft.Json
         /// Writes a comment <c>/*...*/</c> containing the specified text. 
         /// </summary>
         /// <param name="text">Text to place inside the comment.</param>
-        public override void WriteComment(string text)
+        public override void WriteComment(string? text)
         {
             InternalWriteComment();
 
@@ -839,6 +842,7 @@ namespace Newtonsoft.Json
             }
 
             EnsureWriteBuffer();
+            Debug.Assert(_writeBuffer != null);
 
             int totalLength = MathUtils.IntLength(value);
 
@@ -890,6 +894,7 @@ namespace Newtonsoft.Json
         private int WriteNumberToBuffer(uint value, bool negative)
         {
             EnsureWriteBuffer();
+            Debug.Assert(_writeBuffer != null);
 
             int totalLength = MathUtils.IntLength(value);
 

Src/Newtonsoft.Json/JsonValidatingReader.cs

@@ -41,6 +41,8 @@ using System.Linq;
 
 #endif
 
+#nullable disable
+
 namespace Newtonsoft.Json
 {
     /// <summary>

Src/Newtonsoft.Json/JsonWriter.Async.cs

@@ -225,7 +225,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteRawAsync(string json, CancellationToken cancellationToken = default)
+        public virtual Task WriteRawAsync(string? json, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -546,7 +546,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteCommentAsync(string text, CancellationToken cancellationToken = default)
+        public virtual Task WriteCommentAsync(string? text, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -570,7 +570,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteRawValueAsync(string json, CancellationToken cancellationToken = default)
+        public virtual Task WriteRawValueAsync(string? json, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -673,7 +673,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public Task WriteTokenAsync(JsonToken token, object value, CancellationToken cancellationToken = default)
+        public Task WriteTokenAsync(JsonToken token, object? value, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -755,7 +755,7 @@ namespace Newtonsoft.Json
                         return WriteValueAsync(guid, cancellationToken);
                     }
 
-                    return WriteValueAsync((byte[])value, cancellationToken);
+                    return WriteValueAsync((byte[]?)value, cancellationToken);
                 default:
                     throw MiscellaneousUtils.CreateArgumentOutOfRangeException(nameof(token), token, "Unexpected token type.");
             }
@@ -768,7 +768,7 @@ namespace Newtonsoft.Json
             do
             {
                 // write a JValue date when the constructor is for a date
-                if (writeDateConstructorAsDate && reader.TokenType == JsonToken.StartConstructor && string.Equals(reader.Value.ToString(), "Date", StringComparison.Ordinal))
+                if (writeDateConstructorAsDate && reader.TokenType == JsonToken.StartConstructor && string.Equals(reader.Value?.ToString(), "Date", StringComparison.Ordinal))
                 {
                     await WriteConstructorDateAsync(reader, cancellationToken).ConfigureAwait(false);
                 }
@@ -801,7 +801,7 @@ namespace Newtonsoft.Json
             do
             {
                 // write a JValue date when the constructor is for a date
-                if (reader.TokenType == JsonToken.StartConstructor && string.Equals(reader.Value.ToString(), "Date", StringComparison.Ordinal))
+                if (reader.TokenType == JsonToken.StartConstructor && string.Equals(reader.Value?.ToString(), "Date", StringComparison.Ordinal))
                 {
                     WriteConstructorDate(reader);
                 }
@@ -831,7 +831,7 @@ namespace Newtonsoft.Json
                 throw JsonWriterException.Create(this, "Unexpected token when reading date constructor. Expected Integer, got " + reader.TokenType, null);
             }
 
-            DateTime date = DateTimeUtils.ConvertJavaScriptTicksToDateTime((long)reader.Value);
+            DateTime date = DateTimeUtils.ConvertJavaScriptTicksToDateTime((long)reader.Value!);
 
             if (!await reader.ReadAsync(cancellationToken).ConfigureAwait(false))
             {
@@ -929,7 +929,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteValueAsync(byte[] value, CancellationToken cancellationToken = default)
+        public virtual Task WriteValueAsync(byte[]? value, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -1290,7 +1290,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteValueAsync(object value, CancellationToken cancellationToken = default)
+        public virtual Task WriteValueAsync(object? value, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -1387,7 +1387,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteValueAsync(string value, CancellationToken cancellationToken = default)
+        public virtual Task WriteValueAsync(string? value, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -1524,7 +1524,7 @@ namespace Newtonsoft.Json
         /// <returns>A <see cref="Task"/> that represents the asynchronous operation.</returns>
         /// <remarks>The default behaviour is to execute synchronously, returning an already-completed task. Derived
         /// classes can override this behaviour for true asynchronicity.</remarks>
-        public virtual Task WriteValueAsync(Uri value, CancellationToken cancellationToken = default)
+        public virtual Task WriteValueAsync(Uri? value, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {

Src/Newtonsoft.Json/JsonWriter.cs

@@ -116,7 +116,7 @@ namespace Newtonsoft.Json
             StateArray = BuildStateArray();
         }
 
-        private List<JsonPosition> _stack;
+        private List<JsonPosition>? _stack;
         private JsonPosition _currentPosition;
         private State _currentState;
         private Formatting _formatting;
@@ -218,7 +218,7 @@ namespace Newtonsoft.Json
 
                 JsonPosition? current = insideContainer ? (JsonPosition?)_currentPosition : null;
 
-                return JsonPosition.BuildPath(_stack, current);
+                return JsonPosition.BuildPath(_stack!, current);
             }
         }
 
@@ -226,8 +226,8 @@ namespace Newtonsoft.Json
         private DateTimeZoneHandling _dateTimeZoneHandling;
         private StringEscapeHandling _stringEscapeHandling;
         private FloatFormatHandling _floatFormatHandling;
-        private string _dateFormatString;
-        private CultureInfo _culture;
+        private string? _dateFormatString;
+        private CultureInfo? _culture;
 
         /// <summary>
         /// Gets or sets a value indicating how JSON text output should be formatted.
@@ -325,7 +325,7 @@ namespace Newtonsoft.Json
         /// <summary>
         /// Gets or sets how <see cref="DateTime"/> and <see cref="DateTimeOffset"/> values are formatted when writing JSON text.
         /// </summary>
-        public string DateFormatString
+        public string? DateFormatString
         {
             get => _dateFormatString;
             set => _dateFormatString = value;
@@ -522,7 +522,7 @@ namespace Newtonsoft.Json
         /// A value is only required for tokens that have an associated value, e.g. the <see cref="String"/> property name for <see cref="JsonToken.PropertyName"/>.
         /// <c>null</c> can be passed to the method for tokens that don't have a value, e.g. <see cref="JsonToken.StartObject"/>.
         /// </param>
-        public void WriteToken(JsonToken token, object value)
+        public void WriteToken(JsonToken token, object? value)
         {
             switch (token)
             {
@@ -625,7 +625,7 @@ namespace Newtonsoft.Json
                     }
                     else
                     {
-                        WriteValue((byte[])value);
+                        WriteValue((byte[])value!);
                     }
                     break;
                 default:
@@ -649,7 +649,7 @@ namespace Newtonsoft.Json
             do
             {
                 // write a JValue date when the constructor is for a date
-                if (writeDateConstructorAsDate && reader.TokenType == JsonToken.StartConstructor && string.Equals(reader.Value.ToString(), "Date", StringComparison.Ordinal))
+                if (writeDateConstructorAsDate && reader.TokenType == JsonToken.StartConstructor && string.Equals(reader.Value?.ToString(), "Date", StringComparison.Ordinal))
                 {
                     WriteConstructorDate(reader);
                 }
@@ -696,7 +696,7 @@ namespace Newtonsoft.Json
 
         private void WriteConstructorDate(JsonReader reader)
         {
-            if (!JavaScriptUtils.TryGetDateFromConstructorJson(reader, out DateTime dateTime, out string errorMessage))
+            if (!JavaScriptUtils.TryGetDateFromConstructorJson(reader, out DateTime dateTime, out string? errorMessage))
             {
                 throw JsonWriterException.Create(this, errorMessage, null);
             }
@@ -787,7 +787,7 @@ namespace Newtonsoft.Json
                 {
                     int currentLevel = top - i;
 
-                    if (_stack[currentLevel].Type == type)
+                    if (_stack![currentLevel].Type == type)
                     {
                         levelsToComplete = i + 2;
                         break;
@@ -909,7 +909,7 @@ namespace Newtonsoft.Json
         /// Writes raw JSON without changing the writer's state.
         /// </summary>
         /// <param name="json">The raw JSON to write.</param>
-        public virtual void WriteRaw(string json)
+        public virtual void WriteRaw(string? json)
         {
             InternalWriteRaw();
         }
@@ -918,7 +918,7 @@ namespace Newtonsoft.Json
         /// Writes raw JSON where a value is expected and updates the writer's state.
         /// </summary>
         /// <param name="json">The raw JSON to write.</param>
-        public virtual void WriteRawValue(string json)
+        public virtual void WriteRawValue(string? json)
         {
             // hack. want writer to change state as if a value had been written
             UpdateScopeWithFinishedValue();
@@ -930,7 +930,7 @@ namespace Newtonsoft.Json
         /// Writes a <see cref="String"/> value.
         /// </summary>
         /// <param name="value">The <see cref="String"/> value to write.</param>
-        public virtual void WriteValue(string value)
+        public virtual void WriteValue(string? value)
         {
             InternalWriteValue(JsonToken.String);
         }
@@ -1376,7 +1376,7 @@ namespace Newtonsoft.Json
         /// Writes a <see cref="Byte"/>[] value.
         /// </summary>
         /// <param name="value">The <see cref="Byte"/>[] value to write.</param>
-        public virtual void WriteValue(byte[] value)
+        public virtual void WriteValue(byte[]? value)
         {
             if (value == null)
             {
@@ -1392,7 +1392,7 @@ namespace Newtonsoft.Json
         /// Writes a <see cref="Uri"/> value.
         /// </summary>
         /// <param name="value">The <see cref="Uri"/> value to write.</param>
-        public virtual void WriteValue(Uri value)
+        public virtual void WriteValue(Uri? value)
         {
             if (value == null)
             {
@@ -1409,7 +1409,7 @@ namespace Newtonsoft.Json
         /// An error will raised if the value cannot be written as a single JSON token.
         /// </summary>
         /// <param name="value">The <see cref="Object"/> value to write.</param>
-        public virtual void WriteValue(object value)
+        public virtual void WriteValue(object? value)
         {
             if (value == null)
             {
@@ -1435,7 +1435,7 @@ namespace Newtonsoft.Json
         /// Writes a comment <c>/*...*/</c> containing the specified text.
         /// </summary>
         /// <param name="text">Text to place inside the comment.</param>
-        public virtual void WriteComment(string text)
+        public virtual void WriteComment(string? text)
         {
             InternalWriteComment();
         }

Src/Newtonsoft.Json/JsonWriterException.cs

@@ -42,7 +42,7 @@ namespace Newtonsoft.Json
         /// Gets the path to the JSON where the error occurred.
         /// </summary>
         /// <value>The path to the JSON where the error occurred.</value>
-        public string Path { get; }
+        public string? Path { get; }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonWriterException"/> class.
@@ -93,18 +93,18 @@ namespace Newtonsoft.Json
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="path">The path to the JSON where the error occurred.</param>
         /// <param name="innerException">The exception that is the cause of the current exception, or <c>null</c> if no inner exception is specified.</param>
-        public JsonWriterException(string message, string path, Exception innerException)
+        public JsonWriterException(string message, string path, Exception? innerException)
             : base(message, innerException)
         {
             Path = path;
         }
 
-        internal static JsonWriterException Create(JsonWriter writer, string message, Exception ex)
+        internal static JsonWriterException Create(JsonWriter writer, string message, Exception? ex)
         {
             return Create(writer.ContainerPath, message, ex);
         }
 
-        internal static JsonWriterException Create(string path, string message, Exception ex)
+        internal static JsonWriterException Create(string path, string message, Exception? ex)
         {
             message = JsonPosition.FormatMessage(null, path, message);
 

Src/Newtonsoft.Json/Linq/Extensions.cs

@@ -27,6 +27,8 @@ using System;
 using System.Collections.Generic;
 using Newtonsoft.Json.Utilities;
 using System.Globalization;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 #if !HAVE_LINQ
 using Newtonsoft.Json.Utilities.LinqBridge;
 #else
@@ -111,7 +113,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="source">An <see cref="IEnumerable{T}"/> of <see cref="JToken"/> that contains the source collection.</param>
         /// <param name="key">The token key.</param>
         /// <returns>An <see cref="IEnumerable{T}"/> of <see cref="JToken"/> that contains the values of every token in the source collection with the given key.</returns>
-        public static IJEnumerable<JToken> Values(this IEnumerable<JToken> source, object key)
+        public static IJEnumerable<JToken> Values(this IEnumerable<JToken> source, object? key)
         {
             return Values<JToken, JToken>(source, key).AsJEnumerable();
         }
@@ -179,7 +181,7 @@ namespace Newtonsoft.Json.Linq
             return token.Convert<JToken, U>();
         }
 
-        internal static IEnumerable<U> Values<T, U>(this IEnumerable<T> source, object key) where T : JToken
+        internal static IEnumerable<U> Values<T, U>(this IEnumerable<T> source, object? key) where T : JToken
         {
             ValidationUtils.ArgumentNotNull(source, nameof(source));
 
@@ -204,7 +206,7 @@ namespace Newtonsoft.Json.Linq
             {
                 foreach (T token in source)
                 {
-                    JToken value = token[key];
+                    JToken? value = token[key];
                     if (value != null)
                     {
                         yield return value.Convert<JToken, U>();
@@ -251,11 +253,14 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        internal static U Convert<T, U>(this T token) where T : JToken
+        [return: MaybeNull]
+        internal static U Convert<T, U>(this T token) where T : JToken?
         {
             if (token == null)
             {
+#pragma warning disable CS8653 // A default expression introduces a null value for a type parameter.
                 return default;
+#pragma warning restore CS8653 // A default expression introduces a null value for a type parameter.
             }
 
             if (token is U castValue
@@ -282,7 +287,9 @@ namespace Newtonsoft.Json.Linq
                 {
                     if (value.Value == null)
                     {
+#pragma warning disable CS8653 // A default expression introduces a null value for a type parameter.
                         return default;
+#pragma warning restore CS8653 // A default expression introduces a null value for a type parameter.
                     }
 
                     targetType = Nullable.GetUnderlyingType(targetType);
@@ -315,7 +322,7 @@ namespace Newtonsoft.Json.Linq
         {
             if (source == null)
             {
-                return null;
+                return null!;
             }
             else if (source is IJEnumerable<T> customEnumerable)
             {

Src/Newtonsoft.Json/Linq/JArray.Async.cs

@@ -73,7 +73,7 @@ namespace Newtonsoft.Json.Linq
         /// If this is <c>null</c>, default load settings will be used.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation requests. The default value is <see cref="CancellationToken.None"/>.</param>
         /// <returns>A <see cref="Task{TResult}"/> representing the asynchronous load. The <see cref="Task{TResult}.Result"/> property contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static async Task<JArray> LoadAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        public new static async Task<JArray> LoadAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
             if (reader.TokenType == JsonToken.None)
             {

Src/Newtonsoft.Json/Linq/JArray.cs

@@ -115,7 +115,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="settings">The <see cref="JsonLoadSettings"/> used to load the JSON.
         /// If this is <c>null</c>, default load settings will be used.</param>
         /// <returns>A <see cref="JArray"/> that contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static JArray Load(JsonReader reader, JsonLoadSettings settings)
+        public new static JArray Load(JsonReader reader, JsonLoadSettings? settings)
         {
             if (reader.TokenType == JsonToken.None)
             {
@@ -163,7 +163,7 @@ namespace Newtonsoft.Json.Linq
         /// <example>
         ///   <code lang="cs" source="..\Src\Newtonsoft.Json.Tests\Documentation\LinqToJsonTests.cs" region="LinqToJsonCreateParseArray" title="Parsing a JSON Array from Text" />
         /// </example>
-        public new static JArray Parse(string json, JsonLoadSettings settings)
+        public new static JArray Parse(string json, JsonLoadSettings? settings)
         {
             using (JsonReader reader = new JsonTextReader(new StringReader(json)))
             {
@@ -227,7 +227,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets the <see cref="JToken"/> with the specified key.
         /// </summary>
         /// <value>The <see cref="JToken"/> with the specified key.</value>
-        public override JToken this[object key]
+        public override JToken? this[object key]
         {
             get
             {
@@ -263,14 +263,19 @@ namespace Newtonsoft.Json.Linq
             set => SetItem(index, value);
         }
 
-        internal override int IndexOfItem(JToken item)
+        internal override int IndexOfItem(JToken? item)
         {
+            if (item == null)
+            {
+                return -1;
+            }
+
             return _values.IndexOfReference(item);
         }
 
-        internal override void MergeItem(object content, JsonMergeSettings settings)
+        internal override void MergeItem(object content, JsonMergeSettings? settings)
         {
-            IEnumerable a = (IsMultiContent(content) || content is JArray)
+            IEnumerable? a = (IsMultiContent(content) || content is JArray)
                 ? (IEnumerable)content
                 : null;
             if (a == null)

Src/Newtonsoft.Json/Linq/JConstructor.Async.cs

@@ -43,7 +43,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>A <see cref="Task"/> that represents the asynchronous write operation.</returns>
         public override async Task WriteToAsync(JsonWriter writer, CancellationToken cancellationToken, params JsonConverter[] converters)
         {
-            await writer.WriteStartConstructorAsync(_name, cancellationToken).ConfigureAwait(false);
+            await writer.WriteStartConstructorAsync(_name ?? string.Empty, cancellationToken).ConfigureAwait(false);
 
             for (int i = 0; i < _values.Count; i++)
             {
@@ -76,7 +76,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>
         /// A <see cref="Task{TResult}"/> that represents the asynchronous load. The <see cref="Task{TResult}.Result"/>
         /// property returns a <see cref="JConstructor"/> that contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static async Task<JConstructor> LoadAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        public new static async Task<JConstructor> LoadAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
             if (reader.TokenType == JsonToken.None)
             {
@@ -93,7 +93,7 @@ namespace Newtonsoft.Json.Linq
                 throw JsonReaderException.Create(reader, "Error reading JConstructor from JsonReader. Current JsonReader item is not a constructor: {0}".FormatWith(CultureInfo.InvariantCulture, reader.TokenType));
             }
 
-            JConstructor c = new JConstructor((string)reader.Value);
+            JConstructor c = new JConstructor((string)reader.Value!);
             c.SetLineInfo(reader as IJsonLineInfo, settings);
 
             await c.ReadTokenFromAsync(reader, settings, cancellationToken).ConfigureAwait(false);

Src/Newtonsoft.Json/Linq/JConstructor.cs

@@ -36,7 +36,7 @@ namespace Newtonsoft.Json.Linq
     /// </summary>
     public partial class JConstructor : JContainer
     {
-        private string _name;
+        private string? _name;
         private readonly List<JToken> _values = new List<JToken>();
 
         /// <summary>
@@ -45,12 +45,17 @@ namespace Newtonsoft.Json.Linq
         /// <value>The container's children tokens.</value>
         protected override IList<JToken> ChildrenTokens => _values;
 
-        internal override int IndexOfItem(JToken item)
+        internal override int IndexOfItem(JToken? item)
         {
+            if (item == null)
+            {
+                return -1;
+            }
+
             return _values.IndexOfReference(item);
         }
 
-        internal override void MergeItem(object content, JsonMergeSettings settings)
+        internal override void MergeItem(object content, JsonMergeSettings? settings)
         {
             if (!(content is JConstructor c))
             {
@@ -68,7 +73,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets or sets the name of this constructor.
         /// </summary>
         /// <value>The constructor name.</value>
-        public string Name
+        public string? Name
         {
             get => _name;
             set => _name = value;
@@ -154,7 +159,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="converters">A collection of <see cref="JsonConverter"/> which will be used when writing the token.</param>
         public override void WriteTo(JsonWriter writer, params JsonConverter[] converters)
         {
-            writer.WriteStartConstructor(_name);
+            writer.WriteStartConstructor(_name!);
 
             int count = _values.Count;
             for (int i = 0; i < count; i++)
@@ -169,7 +174,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets the <see cref="JToken"/> with the specified key.
         /// </summary>
         /// <value>The <see cref="JToken"/> with the specified key.</value>
-        public override JToken this[object key]
+        public override JToken? this[object key]
         {
             get
             {
@@ -197,7 +202,7 @@ namespace Newtonsoft.Json.Linq
 
         internal override int GetDeepHashCode()
         {
-            return _name.GetHashCode() ^ ContentsHashCode();
+            return (_name?.GetHashCode() ?? 0) ^ ContentsHashCode();
         }
 
         /// <summary>
@@ -217,7 +222,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="settings">The <see cref="JsonLoadSettings"/> used to load the JSON.
         /// If this is <c>null</c>, default load settings will be used.</param>
         /// <returns>A <see cref="JConstructor"/> that contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static JConstructor Load(JsonReader reader, JsonLoadSettings settings)
+        public new static JConstructor Load(JsonReader reader, JsonLoadSettings? settings)
         {
             if (reader.TokenType == JsonToken.None)
             {
@@ -234,7 +239,7 @@ namespace Newtonsoft.Json.Linq
                 throw JsonReaderException.Create(reader, "Error reading JConstructor from JsonReader. Current JsonReader item is not a constructor: {0}".FormatWith(CultureInfo.InvariantCulture, reader.TokenType));
             }
 
-            JConstructor c = new JConstructor((string)reader.Value);
+            JConstructor c = new JConstructor((string)reader.Value!);
             c.SetLineInfo(reader as IJsonLineInfo, settings);
 
             c.ReadTokenFrom(reader, settings);

Src/Newtonsoft.Json/Linq/JContainer.Async.cs

@@ -26,6 +26,7 @@
 #if HAVE_ASYNC
 
 using System;
+using System.Diagnostics;
 using System.Globalization;
 using System.Threading;
 using System.Threading.Tasks;
@@ -35,7 +36,7 @@ namespace Newtonsoft.Json.Linq
 {
     public abstract partial class JContainer
     {
-        internal async Task ReadTokenFromAsync(JsonReader reader, JsonLoadSettings options, CancellationToken cancellationToken = default)
+        internal async Task ReadTokenFromAsync(JsonReader reader, JsonLoadSettings? options, CancellationToken cancellationToken = default)
         {
             ValidationUtils.ArgumentNotNull(reader, nameof(reader));
             int startDepth = reader.Depth;
@@ -53,11 +54,11 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        private async Task ReadContentFromAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        private async Task ReadContentFromAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
-            IJsonLineInfo lineInfo = reader as IJsonLineInfo;
+            IJsonLineInfo? lineInfo = reader as IJsonLineInfo;
 
-            JContainer parent = this;
+            JContainer? parent = this;
 
             do
             {
@@ -71,6 +72,8 @@ namespace Newtonsoft.Json.Linq
                     parent = parent.Parent;
                 }
 
+                Debug.Assert(parent != null);
+
                 switch (reader.TokenType)
                 {
                     case JsonToken.None:
@@ -106,7 +109,7 @@ namespace Newtonsoft.Json.Linq
                         parent = parent.Parent;
                         break;
                     case JsonToken.StartConstructor:
-                        JConstructor constructor = new JConstructor(reader.Value.ToString());
+                        JConstructor constructor = new JConstructor(reader.Value!.ToString());
                         constructor.SetLineInfo(lineInfo, settings);
                         parent.Add(constructor);
                         parent = constructor;
@@ -132,7 +135,7 @@ namespace Newtonsoft.Json.Linq
                     case JsonToken.Comment:
                         if (settings != null && settings.CommentHandling == CommentHandling.Load)
                         {
-                            v = JValue.CreateComment(reader.Value.ToString());
+                            v = JValue.CreateComment(reader.Value!.ToString());
                             v.SetLineInfo(lineInfo, settings);
                             parent.Add(v);
                         }
@@ -148,7 +151,7 @@ namespace Newtonsoft.Json.Linq
                         parent.Add(v);
                         break;
                     case JsonToken.PropertyName:
-                        JProperty property = ReadProperty(reader, settings, lineInfo, parent);
+                        JProperty? property = ReadProperty(reader, settings, lineInfo, parent);
                         if (property != null)
                         {
                             parent = property;

Src/Newtonsoft.Json/Linq/JContainer.cs

@@ -33,11 +33,13 @@ using Newtonsoft.Json.Utilities;
 using System.Collections;
 using System.Globalization;
 using System.ComponentModel;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 #if !HAVE_LINQ
 using Newtonsoft.Json.Utilities.LinqBridge;
 #else
 using System.Linq;
-
 #endif
 
 namespace Newtonsoft.Json.Linq
@@ -55,8 +57,8 @@ namespace Newtonsoft.Json.Linq
 #endif
     {
 #if HAVE_COMPONENT_MODEL
-        internal ListChangedEventHandler _listChanged;
-        internal AddingNewEventHandler _addingNew;
+        internal ListChangedEventHandler? _listChanged;
+        internal AddingNewEventHandler? _addingNew;
 
         /// <summary>
         /// Occurs when the list changes or an item in the list changes.
@@ -77,7 +79,7 @@ namespace Newtonsoft.Json.Linq
         }
 #endif
 #if HAVE_INOTIFY_COLLECTION_CHANGED
-        internal NotifyCollectionChangedEventHandler _collectionChanged;
+        internal NotifyCollectionChangedEventHandler? _collectionChanged;
 
         /// <summary>
         /// Occurs when the items list of the collection has changed, or the collection is reset.
@@ -95,7 +97,7 @@ namespace Newtonsoft.Json.Linq
         /// <value>The container's children tokens.</value>
         protected abstract IList<JToken> ChildrenTokens { get; }
 
-        private object _syncRoot;
+        private object? _syncRoot;
 #if (HAVE_COMPONENT_MODEL || HAVE_INOTIFY_COLLECTION_CHANGED)
         private bool _busy;
 #endif
@@ -148,7 +150,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="e">The <see cref="ListChangedEventArgs"/> instance containing the event data.</param>
         protected virtual void OnListChanged(ListChangedEventArgs e)
         {
-            ListChangedEventHandler handler = _listChanged;
+            ListChangedEventHandler? handler = _listChanged;
 
             if (handler != null)
             {
@@ -171,7 +173,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="e">The <see cref="NotifyCollectionChangedEventArgs"/> instance containing the event data.</param>
         protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
         {
-            NotifyCollectionChangedEventHandler handler = _collectionChanged;
+            NotifyCollectionChangedEventHandler? handler = _collectionChanged;
 
             if (handler != null)
             {
@@ -228,7 +230,7 @@ namespace Newtonsoft.Json.Linq
         /// <value>
         /// A <see cref="JToken"/> containing the first child token of the <see cref="JToken"/>.
         /// </value>
-        public override JToken First
+        public override JToken? First
         {
             get
             {
@@ -243,7 +245,7 @@ namespace Newtonsoft.Json.Linq
         /// <value>
         /// A <see cref="JToken"/> containing the last child token of the <see cref="JToken"/>.
         /// </value>
-        public override JToken Last
+        public override JToken? Last
         {
             get
             {
@@ -314,12 +316,12 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        internal bool IsMultiContent(object content)
+        internal bool IsMultiContent([NotNull]object? content)
         {
             return (content is IEnumerable && !(content is string) && !(content is JToken) && !(content is byte[]));
         }
 
-        internal JToken EnsureParentToken(JToken item, bool skipParentCheck)
+        internal JToken EnsureParentToken(JToken? item, bool skipParentCheck)
         {
             if (item == null)
             {
@@ -343,9 +345,9 @@ namespace Newtonsoft.Json.Linq
             return item;
         }
 
-        internal abstract int IndexOfItem(JToken item);
+        internal abstract int IndexOfItem(JToken? item);
 
-        internal virtual void InsertItem(int index, JToken item, bool skipParentCheck)
+        internal virtual void InsertItem(int index, JToken? item, bool skipParentCheck)
         {
             IList<JToken> children = ChildrenTokens;
 
@@ -358,9 +360,9 @@ namespace Newtonsoft.Json.Linq
 
             item = EnsureParentToken(item, skipParentCheck);
 
-            JToken previous = (index == 0) ? null : children[index - 1];
+            JToken? previous = (index == 0) ? null : children[index - 1];
             // haven't inserted new token yet so next token is still at the inserting index
-            JToken next = (index == children.Count) ? null : children[index];
+            JToken? next = (index == children.Count) ? null : children[index];
 
             ValidateToken(item, null);
 
@@ -410,8 +412,8 @@ namespace Newtonsoft.Json.Linq
             CheckReentrancy();
 
             JToken item = children[index];
-            JToken previous = (index == 0) ? null : children[index - 1];
-            JToken next = (index == children.Count - 1) ? null : children[index + 1];
+            JToken? previous = (index == 0) ? null : children[index - 1];
+            JToken? next = (index == children.Count - 1) ? null : children[index + 1];
 
             if (previous != null)
             {
@@ -442,13 +444,16 @@ namespace Newtonsoft.Json.Linq
 #endif
         }
 
-        internal virtual bool RemoveItem(JToken item)
+        internal virtual bool RemoveItem(JToken? item)
         {
-            int index = IndexOfItem(item);
-            if (index >= 0)
+            if (item != null)
             {
-                RemoveItemAt(index);
-                return true;
+                int index = IndexOfItem(item);
+                if (index >= 0)
+                {
+                    RemoveItemAt(index);
+                    return true;
+                }
             }
 
             return false;
@@ -459,7 +464,7 @@ namespace Newtonsoft.Json.Linq
             return ChildrenTokens[index];
         }
 
-        internal virtual void SetItem(int index, JToken item)
+        internal virtual void SetItem(int index, JToken? item)
         {
             IList<JToken> children = ChildrenTokens;
 
@@ -485,8 +490,8 @@ namespace Newtonsoft.Json.Linq
 
             ValidateToken(item, existing);
 
-            JToken previous = (index == 0) ? null : children[index - 1];
-            JToken next = (index == children.Count - 1) ? null : children[index + 1];
+            JToken? previous = (index == 0) ? null : children[index - 1];
+            JToken? next = (index == children.Count - 1) ? null : children[index + 1];
 
             item.Parent = this;
 
@@ -562,7 +567,7 @@ namespace Newtonsoft.Json.Linq
             SetItem(index, replacement);
         }
 
-        internal virtual bool ContainsItem(JToken item)
+        internal virtual bool ContainsItem(JToken? item)
         {
             return (IndexOfItem(item) != -1);
         }
@@ -594,14 +599,14 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        internal static bool IsTokenUnchanged(JToken currentValue, JToken newValue)
+        internal static bool IsTokenUnchanged(JToken currentValue, JToken? newValue)
         {
             if (currentValue is JValue v1)
             {
-                // null will get turned into a JValue of type null
-                if (v1.Type == JTokenType.Null && newValue == null)
+                if (newValue == null)
                 {
-                    return true;
+                    // null will get turned into a JValue of type null
+                    return v1.Type == JTokenType.Null;
                 }
 
                 return v1.Equals(newValue);
@@ -610,7 +615,7 @@ namespace Newtonsoft.Json.Linq
             return false;
         }
 
-        internal virtual void ValidateToken(JToken o, JToken existing)
+        internal virtual void ValidateToken(JToken o, JToken? existing)
         {
             ValidationUtils.ArgumentNotNull(o, nameof(o));
 
@@ -624,7 +629,7 @@ namespace Newtonsoft.Json.Linq
         /// Adds the specified content as children of this <see cref="JToken"/>.
         /// </summary>
         /// <param name="content">The content to be added.</param>
-        public virtual void Add(object content)
+        public virtual void Add(object? content)
         {
             AddInternal(ChildrenTokens.Count, content, false);
         }
@@ -638,12 +643,12 @@ namespace Newtonsoft.Json.Linq
         /// Adds the specified content as the first children of this <see cref="JToken"/>.
         /// </summary>
         /// <param name="content">The content to be added.</param>
-        public void AddFirst(object content)
+        public void AddFirst(object? content)
         {
             AddInternal(0, content, false);
         }
 
-        internal void AddInternal(int index, object content, bool skipParentCheck)
+        internal void AddInternal(int index, object? content, bool skipParentCheck)
         {
             if (IsMultiContent(content))
             {
@@ -664,7 +669,7 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        internal static JToken CreateFromContent(object content)
+        internal static JToken CreateFromContent(object? content)
         {
             if (content is JToken token)
             {
@@ -701,7 +706,7 @@ namespace Newtonsoft.Json.Linq
             ClearItems();
         }
 
-        internal abstract void MergeItem(object content, JsonMergeSettings settings);
+        internal abstract void MergeItem(object content, JsonMergeSettings? settings);
 
         /// <summary>
         /// Merge the specified content into this <see cref="JToken"/>.
@@ -709,7 +714,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="content">The content to be merged.</param>
         public void Merge(object content)
         {
-            MergeItem(content, new JsonMergeSettings());
+            MergeItem(content, null);
         }
 
         /// <summary>
@@ -717,12 +722,12 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="content">The content to be merged.</param>
         /// <param name="settings">The <see cref="JsonMergeSettings"/> used to merge the content.</param>
-        public void Merge(object content, JsonMergeSettings settings)
+        public void Merge(object content, JsonMergeSettings? settings)
         {
             MergeItem(content, settings);
         }
 
-        internal void ReadTokenFrom(JsonReader reader, JsonLoadSettings options)
+        internal void ReadTokenFrom(JsonReader reader, JsonLoadSettings? options)
         {
             int startDepth = reader.Depth;
 
@@ -741,12 +746,12 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        internal void ReadContentFrom(JsonReader r, JsonLoadSettings settings)
+        internal void ReadContentFrom(JsonReader r, JsonLoadSettings? settings)
         {
             ValidationUtils.ArgumentNotNull(r, nameof(r));
-            IJsonLineInfo lineInfo = r as IJsonLineInfo;
+            IJsonLineInfo? lineInfo = r as IJsonLineInfo;
 
-            JContainer parent = this;
+            JContainer? parent = this;
 
             do
             {
@@ -760,6 +765,8 @@ namespace Newtonsoft.Json.Linq
                     parent = parent.Parent;
                 }
 
+                Debug.Assert(parent != null);
+
                 switch (r.TokenType)
                 {
                     case JsonToken.None:
@@ -795,7 +802,7 @@ namespace Newtonsoft.Json.Linq
                         parent = parent.Parent;
                         break;
                     case JsonToken.StartConstructor:
-                        JConstructor constructor = new JConstructor(r.Value.ToString());
+                        JConstructor constructor = new JConstructor(r.Value!.ToString());
                         constructor.SetLineInfo(lineInfo, settings);
                         parent.Add(constructor);
                         parent = constructor;
@@ -821,7 +828,7 @@ namespace Newtonsoft.Json.Linq
                     case JsonToken.Comment:
                         if (settings != null && settings.CommentHandling == CommentHandling.Load)
                         {
-                            v = JValue.CreateComment(r.Value.ToString());
+                            v = JValue.CreateComment(r.Value!.ToString());
                             v.SetLineInfo(lineInfo, settings);
                             parent.Add(v);
                         }
@@ -837,7 +844,7 @@ namespace Newtonsoft.Json.Linq
                         parent.Add(v);
                         break;
                     case JsonToken.PropertyName:
-                        JProperty property = ReadProperty(r, settings, lineInfo, parent);
+                        JProperty? property = ReadProperty(r, settings, lineInfo, parent);
                         if (property != null)
                         {
                             parent = property;
@@ -853,13 +860,13 @@ namespace Newtonsoft.Json.Linq
             } while (r.Read());
         }
 
-        private static JProperty ReadProperty(JsonReader r, JsonLoadSettings settings, IJsonLineInfo lineInfo, JContainer parent)
+        private static JProperty? ReadProperty(JsonReader r, JsonLoadSettings? settings, IJsonLineInfo? lineInfo, JContainer parent)
         {
             DuplicatePropertyNameHandling duplicatePropertyNameHandling = settings?.DuplicatePropertyNameHandling ?? DuplicatePropertyNameHandling.Replace;
 
             JObject parentObject = (JObject)parent;
-            string propertyName = r.Value.ToString();
-            JProperty existingPropertyWithName = parentObject.Property(propertyName, StringComparison.Ordinal);
+            string propertyName = r.Value!.ToString();
+            JProperty? existingPropertyWithName = parentObject.Property(propertyName, StringComparison.Ordinal);
             if (existingPropertyWithName != null)
             {
                 if (duplicatePropertyNameHandling == DuplicatePropertyNameHandling.Ignore)
@@ -903,9 +910,9 @@ namespace Newtonsoft.Json.Linq
             return string.Empty;
         }
 
-        PropertyDescriptorCollection ITypedList.GetItemProperties(PropertyDescriptor[] listAccessors)
+        PropertyDescriptorCollection? ITypedList.GetItemProperties(PropertyDescriptor[] listAccessors)
         {
-            ICustomTypeDescriptor d = First as ICustomTypeDescriptor;
+            ICustomTypeDescriptor? d = First as ICustomTypeDescriptor;
             return d?.GetProperties();
         }
 #endif
@@ -962,7 +969,7 @@ namespace Newtonsoft.Json.Linq
         }
         #endregion
 
-        private JToken EnsureValue(object value)
+        private JToken? EnsureValue(object value)
         {
             if (value == null)
             {
@@ -1108,7 +1115,7 @@ namespace Newtonsoft.Json.Linq
 
         ListSortDirection IBindingList.SortDirection => ListSortDirection.Ascending;
 
-        PropertyDescriptor IBindingList.SortProperty => null;
+        PropertyDescriptor? IBindingList.SortProperty => null;
 
         bool IBindingList.SupportsChangeNotification => true;
 
@@ -1118,9 +1125,9 @@ namespace Newtonsoft.Json.Linq
 #endif
         #endregion
 
-        internal static void MergeEnumerableContent(JContainer target, IEnumerable content, JsonMergeSettings settings)
+        internal static void MergeEnumerableContent(JContainer target, IEnumerable content, JsonMergeSettings? settings)
         {
-            switch (settings.MergeArrayHandling)
+            switch (settings?.MergeArrayHandling ?? MergeArrayHandling.Concat)
             {
                 case MergeArrayHandling.Concat:
                     foreach (JToken item in content)
@@ -1169,7 +1176,7 @@ namespace Newtonsoft.Json.Linq
                     {
                         if (i < target.Count)
                         {
-                            JToken sourceItem = target[i];
+                            JToken? sourceItem = target[i];
 
                             if (sourceItem is JContainer existingContainer)
                             {

Src/Newtonsoft.Json/Linq/JObject.Async.cs

@@ -97,7 +97,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>
         /// A <see cref="Task{TResult}"/> that represents the asynchronous load. The <see cref="Task{TResult}.Result"/>
         /// property returns a <see cref="JObject"/> that contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static async Task<JObject> LoadAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        public new static async Task<JObject> LoadAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
             ValidationUtils.ArgumentNotNull(reader, nameof(reader));
 

Src/Newtonsoft.Json/Linq/JObject.cs

@@ -37,6 +37,8 @@ using System.Linq.Expressions;
 using System.IO;
 using Newtonsoft.Json.Utilities;
 using System.Globalization;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 #if !HAVE_LINQ
 using Newtonsoft.Json.Utilities.LinqBridge;
 #else
@@ -51,7 +53,7 @@ namespace Newtonsoft.Json.Linq
     /// <example>
     ///   <code lang="cs" source="..\Src\Newtonsoft.Json.Tests\Documentation\LinqToJsonTests.cs" region="LinqToJsonCreateParse" title="Parsing a JSON Object from Text" />
     /// </example>
-    public partial class JObject : JContainer, IDictionary<string, JToken>, INotifyPropertyChanged
+    public partial class JObject : JContainer, IDictionary<string, JToken?>, INotifyPropertyChanged
 #if HAVE_COMPONENT_MODEL
         , ICustomTypeDescriptor
 #endif
@@ -123,12 +125,17 @@ namespace Newtonsoft.Json.Linq
             return _properties.Compare(t._properties);
         }
 
-        internal override int IndexOfItem(JToken item)
+        internal override int IndexOfItem(JToken? item)
         {
+            if (item == null)
+            {
+                return -1;
+            }
+
             return _properties.IndexOfReference(item);
         }
 
-        internal override void InsertItem(int index, JToken item, bool skipParentCheck)
+        internal override void InsertItem(int index, JToken? item, bool skipParentCheck)
         {
             // don't add comments to JObject, no name to reference comment by
             if (item != null && item.Type == JTokenType.Comment)
@@ -139,7 +146,7 @@ namespace Newtonsoft.Json.Linq
             base.InsertItem(index, item, skipParentCheck);
         }
 
-        internal override void ValidateToken(JToken o, JToken existing)
+        internal override void ValidateToken(JToken o, JToken? existing)
         {
             ValidationUtils.ArgumentNotNull(o, nameof(o));
 
@@ -166,16 +173,16 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        internal override void MergeItem(object content, JsonMergeSettings settings)
+        internal override void MergeItem(object content, JsonMergeSettings? settings)
         {
             if (!(content is JObject o))
             {
                 return;
             }
 
-            foreach (KeyValuePair<string, JToken> contentItem in o)
+            foreach (KeyValuePair<string, JToken?> contentItem in o)
             {
-                JProperty existingProperty = Property(contentItem.Key, settings?.PropertyNameComparison ?? StringComparison.Ordinal);
+                JProperty? existingProperty = Property(contentItem.Key, settings?.PropertyNameComparison ?? StringComparison.Ordinal);
 
                 if (existingProperty == null)
                 {
@@ -262,7 +269,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="name">The property name.</param>
         /// <returns>A <see cref="JProperty"/> with the specified name or <c>null</c>.</returns>
-        public JProperty Property(string name)
+        public JProperty? Property(string name)
         {
             return Property(name, StringComparison.Ordinal);
         }
@@ -275,14 +282,14 @@ namespace Newtonsoft.Json.Linq
         /// <param name="name">The property name.</param>
         /// <param name="comparison">One of the enumeration values that specifies how the strings will be compared.</param>
         /// <returns>A <see cref="JProperty"/> matched with the specified name or <c>null</c>.</returns>
-        public JProperty Property(string name, StringComparison comparison)
+        public JProperty? Property(string name, StringComparison comparison)
         {
             if (name == null)
             {
                 return null;
             }
 
-            if (_properties.TryGetValue(name, out JToken property))
+            if (_properties.TryGetValue(name, out JToken? property))
             {
                 return (JProperty)property;
             }
@@ -316,7 +323,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets the <see cref="JToken"/> with the specified key.
         /// </summary>
         /// <value>The <see cref="JToken"/> with the specified key.</value>
-        public override JToken this[object key]
+        public override JToken? this[object key]
         {
             get
             {
@@ -346,29 +353,29 @@ namespace Newtonsoft.Json.Linq
         /// Gets or sets the <see cref="JToken"/> with the specified property name.
         /// </summary>
         /// <value></value>
-        public JToken this[string propertyName]
+        public JToken? this[string propertyName]
         {
             get
             {
                 ValidationUtils.ArgumentNotNull(propertyName, nameof(propertyName));
 
-                JProperty property = Property(propertyName, StringComparison.Ordinal);
+                JProperty? property = Property(propertyName, StringComparison.Ordinal);
 
                 return property?.Value;
             }
             set
             {
-                JProperty property = Property(propertyName, StringComparison.Ordinal);
+                JProperty? property = Property(propertyName, StringComparison.Ordinal);
                 if (property != null)
                 {
-                    property.Value = value;
+                    property.Value = value!;
                 }
                 else
                 {
 #if HAVE_INOTIFY_PROPERTY_CHANGING
                     OnPropertyChanging(propertyName);
 #endif
-                    Add(new JProperty(propertyName, value));
+                    Add(propertyName, value);
                     OnPropertyChanged(propertyName);
                 }
             }
@@ -397,7 +404,7 @@ namespace Newtonsoft.Json.Linq
         /// <exception cref="JsonReaderException">
         ///     <paramref name="reader"/> is not valid JSON.
         /// </exception>
-        public new static JObject Load(JsonReader reader, JsonLoadSettings settings)
+        public new static JObject Load(JsonReader reader, JsonLoadSettings? settings)
         {
             ValidationUtils.ArgumentNotNull(reader, nameof(reader));
 
@@ -453,7 +460,7 @@ namespace Newtonsoft.Json.Linq
         /// <example>
         ///   <code lang="cs" source="..\Src\Newtonsoft.Json.Tests\Documentation\LinqToJsonTests.cs" region="LinqToJsonCreateParse" title="Parsing a JSON Object from Text" />
         /// </example>
-        public new static JObject Parse(string json, JsonLoadSettings settings)
+        public new static JObject Parse(string json, JsonLoadSettings? settings)
         {
             using (JsonReader reader = new JsonTextReader(new StringReader(json)))
             {
@@ -488,7 +495,7 @@ namespace Newtonsoft.Json.Linq
         {
             JToken token = FromObjectInternal(o, jsonSerializer);
 
-            if (token != null && token.Type != JTokenType.Object)
+            if (token.Type != JTokenType.Object)
             {
                 throw new ArgumentException("Object serialized to {0}. JObject instance expected.".FormatWith(CultureInfo.InvariantCulture, token.Type));
             }
@@ -518,7 +525,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="propertyName">Name of the property.</param>
         /// <returns>The <see cref="Newtonsoft.Json.Linq.JToken"/> with the specified property name.</returns>
-        public JToken GetValue(string propertyName)
+        public JToken? GetValue(string? propertyName)
         {
             return GetValue(propertyName, StringComparison.Ordinal);
         }
@@ -531,7 +538,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="propertyName">Name of the property.</param>
         /// <param name="comparison">One of the enumeration values that specifies how the strings will be compared.</param>
         /// <returns>The <see cref="Newtonsoft.Json.Linq.JToken"/> with the specified property name.</returns>
-        public JToken GetValue(string propertyName, StringComparison comparison)
+        public JToken? GetValue(string? propertyName, StringComparison comparison)
         {
             if (propertyName == null)
             {
@@ -553,7 +560,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="value">The value.</param>
         /// <param name="comparison">One of the enumeration values that specifies how the strings will be compared.</param>
         /// <returns><c>true</c> if a value was successfully retrieved; otherwise, <c>false</c>.</returns>
-        public bool TryGetValue(string propertyName, StringComparison comparison, out JToken value)
+        public bool TryGetValue(string propertyName, StringComparison comparison, [NotNullWhen(true)]out JToken? value)
         {
             value = GetValue(propertyName, comparison);
             return (value != null);
@@ -565,7 +572,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="propertyName">Name of the property.</param>
         /// <param name="value">The value.</param>
-        public void Add(string propertyName, JToken value)
+        public void Add(string propertyName, JToken? value)
         {
             Add(new JProperty(propertyName, value));
         }
@@ -582,7 +589,7 @@ namespace Newtonsoft.Json.Linq
             return _properties.Contains(propertyName);
         }
 
-        ICollection<string> IDictionary<string, JToken>.Keys => _properties.Keys;
+        ICollection<string> IDictionary<string, JToken?>.Keys => _properties.Keys;
 
         /// <summary>
         /// Removes the property with the specified name.
@@ -591,7 +598,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns><c>true</c> if item was successfully removed; otherwise, <c>false</c>.</returns>
         public bool Remove(string propertyName)
         {
-            JProperty property = Property(propertyName, StringComparison.Ordinal);
+            JProperty? property = Property(propertyName, StringComparison.Ordinal);
             if (property == null)
             {
                 return false;
@@ -607,9 +614,9 @@ namespace Newtonsoft.Json.Linq
         /// <param name="propertyName">Name of the property.</param>
         /// <param name="value">The value.</param>
         /// <returns><c>true</c> if a value was successfully retrieved; otherwise, <c>false</c>.</returns>
-        public bool TryGetValue(string propertyName, out JToken value)
+        public bool TryGetValue(string propertyName, [NotNullWhen(true)]out JToken? value)
         {
-            JProperty property = Property(propertyName, StringComparison.Ordinal);
+            JProperty? property = Property(propertyName, StringComparison.Ordinal);
             if (property == null)
             {
                 value = null;
@@ -620,24 +627,24 @@ namespace Newtonsoft.Json.Linq
             return true;
         }
 
-        ICollection<JToken> IDictionary<string, JToken>.Values => throw new NotImplementedException();
+        ICollection<JToken?> IDictionary<string, JToken?>.Values => throw new NotImplementedException();
 
         #endregion
 
         #region ICollection<KeyValuePair<string,JToken>> Members
-        void ICollection<KeyValuePair<string, JToken>>.Add(KeyValuePair<string, JToken> item)
+        void ICollection<KeyValuePair<string, JToken?>>.Add(KeyValuePair<string, JToken?> item)
         {
             Add(new JProperty(item.Key, item.Value));
         }
 
-        void ICollection<KeyValuePair<string, JToken>>.Clear()
+        void ICollection<KeyValuePair<string, JToken?>>.Clear()
         {
             RemoveAll();
         }
 
-        bool ICollection<KeyValuePair<string, JToken>>.Contains(KeyValuePair<string, JToken> item)
+        bool ICollection<KeyValuePair<string, JToken?>>.Contains(KeyValuePair<string, JToken?> item)
         {
-            JProperty property = Property(item.Key, StringComparison.Ordinal);
+            JProperty? property = Property(item.Key, StringComparison.Ordinal);
             if (property == null)
             {
                 return false;
@@ -646,7 +653,7 @@ namespace Newtonsoft.Json.Linq
             return (property.Value == item.Value);
         }
 
-        void ICollection<KeyValuePair<string, JToken>>.CopyTo(KeyValuePair<string, JToken>[] array, int arrayIndex)
+        void ICollection<KeyValuePair<string, JToken?>>.CopyTo(KeyValuePair<string, JToken?>[] array, int arrayIndex)
         {
             if (array == null)
             {
@@ -668,16 +675,16 @@ namespace Newtonsoft.Json.Linq
             int index = 0;
             foreach (JProperty property in _properties)
             {
-                array[arrayIndex + index] = new KeyValuePair<string, JToken>(property.Name, property.Value);
+                array[arrayIndex + index] = new KeyValuePair<string, JToken?>(property.Name, property.Value);
                 index++;
             }
         }
 
-        bool ICollection<KeyValuePair<string, JToken>>.IsReadOnly => false;
+        bool ICollection<KeyValuePair<string, JToken?>>.IsReadOnly => false;
 
-        bool ICollection<KeyValuePair<string, JToken>>.Remove(KeyValuePair<string, JToken> item)
+        bool ICollection<KeyValuePair<string, JToken?>>.Remove(KeyValuePair<string, JToken?> item)
         {
-            if (!((ICollection<KeyValuePair<string, JToken>>)this).Contains(item))
+            if (!((ICollection<KeyValuePair<string, JToken?>>)this).Contains(item))
             {
                 return false;
             }
@@ -698,11 +705,11 @@ namespace Newtonsoft.Json.Linq
         /// <returns>
         /// A <see cref="IEnumerator{T}"/> that can be used to iterate through the collection.
         /// </returns>
-        public IEnumerator<KeyValuePair<string, JToken>> GetEnumerator()
+        public IEnumerator<KeyValuePair<string, JToken?>> GetEnumerator()
         {
             foreach (JProperty property in _properties)
             {
-                yield return new KeyValuePair<string, JToken>(property.Name, property.Value);
+                yield return new KeyValuePair<string, JToken?>(property.Name, property.Value);
             }
         }
 
@@ -739,7 +746,7 @@ namespace Newtonsoft.Json.Linq
         {
             PropertyDescriptorCollection descriptors = new PropertyDescriptorCollection(null);
 
-            foreach (KeyValuePair<string, JToken> propertyValue in this)
+            foreach (KeyValuePair<string, JToken?> propertyValue in this)
             {
                 descriptors.Add(new JPropertyDescriptor(propertyValue.Key));
             }
@@ -752,12 +759,12 @@ namespace Newtonsoft.Json.Linq
             return AttributeCollection.Empty;
         }
 
-        string ICustomTypeDescriptor.GetClassName()
+        string? ICustomTypeDescriptor.GetClassName()
         {
             return null;
         }
 
-        string ICustomTypeDescriptor.GetComponentName()
+        string? ICustomTypeDescriptor.GetComponentName()
         {
             return null;
         }
@@ -767,17 +774,17 @@ namespace Newtonsoft.Json.Linq
             return new TypeConverter();
         }
 
-        EventDescriptor ICustomTypeDescriptor.GetDefaultEvent()
+        EventDescriptor? ICustomTypeDescriptor.GetDefaultEvent()
         {
             return null;
         }
 
-        PropertyDescriptor ICustomTypeDescriptor.GetDefaultProperty()
+        PropertyDescriptor? ICustomTypeDescriptor.GetDefaultProperty()
         {
             return null;
         }
 
-        object ICustomTypeDescriptor.GetEditor(Type editorBaseType)
+        object? ICustomTypeDescriptor.GetEditor(Type editorBaseType)
         {
             return null;
         }
@@ -792,7 +799,7 @@ namespace Newtonsoft.Json.Linq
             return EventDescriptorCollection.Empty;
         }
 
-        object ICustomTypeDescriptor.GetPropertyOwner(PropertyDescriptor pd)
+        object? ICustomTypeDescriptor.GetPropertyOwner(PropertyDescriptor pd)
         {
             if (pd is JPropertyDescriptor)
             {
@@ -820,7 +827,7 @@ namespace Newtonsoft.Json.Linq
 
         private class JObjectDynamicProxy : DynamicProxy<JObject>
         {
-            public override bool TryGetMember(JObject instance, GetMemberBinder binder, out object result)
+            public override bool TryGetMember(JObject instance, GetMemberBinder binder, out object? result)
             {
                 // result can be null
                 result = instance[binder.Name];

Src/Newtonsoft.Json/Linq/JProperty.Async.cs

@@ -88,7 +88,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="cancellationToken">The token to monitor for cancellation requests. The default value is <see cref="CancellationToken.None"/>.</param>
         /// <returns>A <see cref="Task{TResult}"/> representing the asynchronous creation. The <see cref="Task{TResult}.Result"/>
         /// property returns a <see cref="JProperty"/> that contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static async Task<JProperty> LoadAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        public new static async Task<JProperty> LoadAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
             if (reader.TokenType == JsonToken.None)
             {
@@ -105,7 +105,7 @@ namespace Newtonsoft.Json.Linq
                 throw JsonReaderException.Create(reader, "Error reading JProperty from JsonReader. Current JsonReader item is not a property: {0}".FormatWith(CultureInfo.InvariantCulture, reader.TokenType));
             }
 
-            JProperty p = new JProperty((string)reader.Value);
+            JProperty p = new JProperty((string)reader.Value!);
             p.SetLineInfo(reader as IJsonLineInfo, settings);
 
             await p.ReadTokenFromAsync(reader, settings, cancellationToken).ConfigureAwait(false);

Src/Newtonsoft.Json/Linq/JProperty.cs

@@ -40,7 +40,7 @@ namespace Newtonsoft.Json.Linq
         #region JPropertyList
         private class JPropertyList : IList<JToken>
         {
-            internal JToken _token;
+            internal JToken? _token;
 
             public IEnumerator<JToken> GetEnumerator()
             {
@@ -115,13 +115,24 @@ namespace Newtonsoft.Json.Linq
 
             public JToken this[int index]
             {
-                get => (index == 0) ? _token : null;
+                get
+                {
+                    if (index != 0)
+                    {
+                        throw new IndexOutOfRangeException();
+                    }
+
+                    Debug.Assert(_token != null);
+                    return _token;
+                }
                 set
                 {
-                    if (index == 0)
+                    if (index != 0)
                     {
-                        _token = value;
+                        throw new IndexOutOfRangeException();
                     }
+
+                    _token = value;
                 }
             }
         }
@@ -153,7 +164,7 @@ namespace Newtonsoft.Json.Linq
         public JToken Value
         {
             [DebuggerStepThrough]
-            get { return _content._token; }
+            get { return _content._token!; }
             set
             {
                 CheckReentrancy();
@@ -191,7 +202,7 @@ namespace Newtonsoft.Json.Linq
             return Value;
         }
 
-        internal override void SetItem(int index, JToken item)
+        internal override void SetItem(int index, JToken? item)
         {
             if (index != 0)
             {
@@ -203,14 +214,14 @@ namespace Newtonsoft.Json.Linq
                 return;
             }
 
-            ((JObject)Parent)?.InternalPropertyChanging(this);
+            ((JObject?)Parent)?.InternalPropertyChanging(this);
 
             base.SetItem(0, item);
 
-            ((JObject)Parent)?.InternalPropertyChanged(this);
+            ((JObject?)Parent)?.InternalPropertyChanged(this);
         }
 
-        internal override bool RemoveItem(JToken item)
+        internal override bool RemoveItem(JToken? item)
         {
             throw new JsonException("Cannot add or remove items from {0}.".FormatWith(CultureInfo.InvariantCulture, typeof(JProperty)));
         }
@@ -220,12 +231,17 @@ namespace Newtonsoft.Json.Linq
             throw new JsonException("Cannot add or remove items from {0}.".FormatWith(CultureInfo.InvariantCulture, typeof(JProperty)));
         }
 
-        internal override int IndexOfItem(JToken item)
+        internal override int IndexOfItem(JToken? item)
         {
+            if (item == null)
+            {
+                return -1;
+            }
+
             return _content.IndexOf(item);
         }
 
-        internal override void InsertItem(int index, JToken item, bool skipParentCheck)
+        internal override void InsertItem(int index, JToken? item, bool skipParentCheck)
         {
             // don't add comments to JProperty
             if (item != null && item.Type == JTokenType.Comment)
@@ -241,14 +257,14 @@ namespace Newtonsoft.Json.Linq
             base.InsertItem(0, item, false);
         }
 
-        internal override bool ContainsItem(JToken item)
+        internal override bool ContainsItem(JToken? item)
         {
             return (Value == item);
         }
 
-        internal override void MergeItem(object content, JsonMergeSettings settings)
+        internal override void MergeItem(object content, JsonMergeSettings? settings)
         {
-            JToken value = (content as JProperty)?.Value;
+            JToken? value = (content as JProperty)?.Value;
 
             if (value != null && value.Type != JTokenType.Null)
             {
@@ -304,7 +320,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="name">The property name.</param>
         /// <param name="content">The property content.</param>
-        public JProperty(string name, object content)
+        public JProperty(string name, object? content)
         {
             ValidationUtils.ArgumentNotNull(name, nameof(name));
 
@@ -357,7 +373,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="settings">The <see cref="JsonLoadSettings"/> used to load the JSON.
         /// If this is <c>null</c>, default load settings will be used.</param>
         /// <returns>A <see cref="JProperty"/> that contains the JSON that was read from the specified <see cref="JsonReader"/>.</returns>
-        public new static JProperty Load(JsonReader reader, JsonLoadSettings settings)
+        public new static JProperty Load(JsonReader reader, JsonLoadSettings? settings)
         {
             if (reader.TokenType == JsonToken.None)
             {
@@ -374,7 +390,7 @@ namespace Newtonsoft.Json.Linq
                 throw JsonReaderException.Create(reader, "Error reading JProperty from JsonReader. Current JsonReader item is not a property: {0}".FormatWith(CultureInfo.InvariantCulture, reader.TokenType));
             }
 
-            JProperty p = new JProperty((string)reader.Value);
+            JProperty p = new JProperty((string)reader.Value!);
             p.SetLineInfo(reader as IJsonLineInfo, settings);
 
             p.ReadTokenFrom(reader, settings);

Src/Newtonsoft.Json/Linq/JPropertyDescriptor.cs

@@ -67,7 +67,7 @@ namespace Newtonsoft.Json.Linq
         /// The value of a property for a given component.
         /// </returns>
         /// <param name="component">The component with the property for which to retrieve the value.</param>
-        public override object GetValue(object component)
+        public override object? GetValue(object component)
         {
             return (component as JObject)?[Name];
         }

Src/Newtonsoft.Json/Linq/JPropertyKeyedCollection.cs

@@ -26,6 +26,8 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using Newtonsoft.Json.Utilities;
 
 namespace Newtonsoft.Json.Linq
@@ -34,7 +36,7 @@ namespace Newtonsoft.Json.Linq
     {
         private static readonly IEqualityComparer<string> Comparer = StringComparer.Ordinal;
 
-        private Dictionary<string, JToken> _dictionary;
+        private Dictionary<string, JToken>? _dictionary;
 
         public JPropertyKeyedCollection() : base(new List<JToken>())
         {
@@ -43,7 +45,7 @@ namespace Newtonsoft.Json.Linq
         private void AddKey(string key, JToken item)
         {
             EnsureDictionary();
-            _dictionary[key] = item;
+            _dictionary![key] = item;
         }
 
         protected void ChangeItemKey(JToken item, string newKey)
@@ -189,7 +191,7 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        public bool TryGetValue(string key, out JToken value)
+        public bool TryGetValue(string key, [NotNullWhen(true)]out JToken? value)
         {
             if (_dictionary == null)
             {
@@ -205,7 +207,7 @@ namespace Newtonsoft.Json.Linq
             get
             {
                 EnsureDictionary();
-                return _dictionary.Keys;
+                return _dictionary!.Keys;
             }
         }
 
@@ -214,7 +216,7 @@ namespace Newtonsoft.Json.Linq
             get
             {
                 EnsureDictionary();
-                return _dictionary.Values;
+                return _dictionary!.Values;
             }
         }
 
@@ -232,8 +234,8 @@ namespace Newtonsoft.Json.Linq
 
             // dictionaries in JavaScript aren't ordered
             // ignore order when comparing properties
-            Dictionary<string, JToken> d1 = _dictionary;
-            Dictionary<string, JToken> d2 = other._dictionary;
+            Dictionary<string, JToken>? d1 = _dictionary;
+            Dictionary<string, JToken>? d2 = other._dictionary;
 
             if (d1 == null && d2 == null)
             {
@@ -242,7 +244,7 @@ namespace Newtonsoft.Json.Linq
 
             if (d1 == null)
             {
-                return (d2.Count == 0);
+                return (d2!.Count == 0);
             }
 
             if (d2 == null)

Src/Newtonsoft.Json/Linq/JRaw.cs

@@ -46,7 +46,7 @@ namespace Newtonsoft.Json.Linq
         /// Initializes a new instance of the <see cref="JRaw"/> class.
         /// </summary>
         /// <param name="rawJson">The raw json.</param>
-        public JRaw(object rawJson)
+        public JRaw(object? rawJson)
             : base(rawJson, JTokenType.Raw)
         {
         }

Src/Newtonsoft.Json/Linq/JToken.Async.cs

@@ -89,7 +89,7 @@ namespace Newtonsoft.Json.Linq
         /// that were read from the reader. The runtime type of the token is determined
         /// by the token type of the first token encountered in the reader.
         /// </returns>
-        public static async Task<JToken> ReadFromAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        public static async Task<JToken> ReadFromAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
             ValidationUtils.ArgumentNotNull(reader, nameof(reader));
 
@@ -101,7 +101,7 @@ namespace Newtonsoft.Json.Linq
                 }
             }
 
-            IJsonLineInfo lineInfo = reader as IJsonLineInfo;
+            IJsonLineInfo? lineInfo = reader as IJsonLineInfo;
 
             switch (reader.TokenType)
             {
@@ -123,7 +123,7 @@ namespace Newtonsoft.Json.Linq
                     v.SetLineInfo(lineInfo, settings);
                     return v;
                 case JsonToken.Comment:
-                    v = JValue.CreateComment(reader.Value.ToString());
+                    v = JValue.CreateComment(reader.Value?.ToString());
                     v.SetLineInfo(lineInfo, settings);
                     return v;
                 case JsonToken.Null:
@@ -168,7 +168,7 @@ namespace Newtonsoft.Json.Linq
         /// that were read from the reader. The runtime type of the token is determined
         /// by the token type of the first token encountered in the reader.
         /// </returns>
-        public static Task<JToken> LoadAsync(JsonReader reader, JsonLoadSettings settings, CancellationToken cancellationToken = default)
+        public static Task<JToken> LoadAsync(JsonReader reader, JsonLoadSettings? settings, CancellationToken cancellationToken = default)
         {
             return ReadFromAsync(reader, settings, cancellationToken);
         }

Src/Newtonsoft.Json/Linq/JToken.cs

@@ -38,11 +38,12 @@ using Newtonsoft.Json.Utilities;
 using System.Diagnostics;
 using System.Globalization;
 using System.Collections;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 #if !HAVE_LINQ
 using Newtonsoft.Json.Utilities.LinqBridge;
 #else
 using System.Linq;
-
 #endif
 
 namespace Newtonsoft.Json.Linq
@@ -60,10 +61,10 @@ namespace Newtonsoft.Json.Linq
     {
         private static JTokenEqualityComparer _equalityComparer;
 
-        private JContainer _parent;
-        private JToken _previous;
-        private JToken _next;
-        private object _annotations;
+        private JContainer? _parent;
+        private JToken? _previous;
+        private JToken? _next;
+        private object? _annotations;
 
         private static readonly JTokenType[] BooleanTypes = new[] { JTokenType.Integer, JTokenType.Float, JTokenType.String, JTokenType.Comment, JTokenType.Raw, JTokenType.Boolean };
         private static readonly JTokenType[] NumberTypes = new[] { JTokenType.Integer, JTokenType.Float, JTokenType.String, JTokenType.Comment, JTokenType.Raw, JTokenType.Boolean };
@@ -99,7 +100,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets or sets the parent.
         /// </summary>
         /// <value>The parent.</value>
-        public JContainer Parent
+        public JContainer? Parent
         {
             [DebuggerStepThrough]
             get { return _parent; }
@@ -114,7 +115,7 @@ namespace Newtonsoft.Json.Linq
         {
             get
             {
-                JContainer parent = Parent;
+                JContainer? parent = Parent;
                 if (parent == null)
                 {
                     return this;
@@ -161,7 +162,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets the next sibling token of this node.
         /// </summary>
         /// <value>The <see cref="JToken"/> that contains the next sibling token.</value>
-        public JToken Next
+        public JToken? Next
         {
             get => _next;
             internal set => _next = value;
@@ -171,7 +172,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets the previous sibling token of this node.
         /// </summary>
         /// <value>The <see cref="JToken"/> that contains the previous sibling token.</value>
-        public JToken Previous
+        public JToken? Previous
         {
             get => _previous;
             internal set => _previous = value;
@@ -190,8 +191,8 @@ namespace Newtonsoft.Json.Linq
                 }
 
                 List<JsonPosition> positions = new List<JsonPosition>();
-                JToken previous = null;
-                for (JToken current = this; current != null; current = current.Parent)
+                JToken? previous = null;
+                for (JToken? current = this; current != null; current = current.Parent)
                 {
                     switch (current.Type)
                     {
@@ -277,7 +278,7 @@ namespace Newtonsoft.Json.Linq
 
         internal IEnumerable<JToken> GetAncestors(bool self)
         {
-            for (JToken current = self ? this : Parent; current != null; current = current.Parent)
+            for (JToken? current = self ? this : Parent; current != null; current = current.Parent)
             {
                 yield return current;
             }
@@ -294,7 +295,7 @@ namespace Newtonsoft.Json.Linq
                 yield break;
             }
 
-            for (JToken o = Next; o != null; o = o.Next)
+            for (JToken? o = Next; o != null; o = o.Next)
             {
                 yield return o;
             }
@@ -306,7 +307,12 @@ namespace Newtonsoft.Json.Linq
         /// <returns>A collection of the sibling tokens before this token, in document order.</returns>
         public IEnumerable<JToken> BeforeSelf()
         {
-            for (JToken o = Parent.First; o != this; o = o.Next)
+            if (Parent == null)
+            {
+                yield break;
+            }
+
+            for (JToken? o = Parent.First; o != this && o != null; o = o.Next)
             {
                 yield return o;
             }
@@ -316,7 +322,7 @@ namespace Newtonsoft.Json.Linq
         /// Gets the <see cref="JToken"/> with the specified key.
         /// </summary>
         /// <value>The <see cref="JToken"/> with the specified key.</value>
-        public virtual JToken this[object key]
+        public virtual JToken? this[object key]
         {
             get => throw new InvalidOperationException("Cannot access child value on {0}.".FormatWith(CultureInfo.InvariantCulture, GetType()));
             set => throw new InvalidOperationException("Cannot set child value on {0}.".FormatWith(CultureInfo.InvariantCulture, GetType()));
@@ -330,7 +336,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The converted token value.</returns>
         public virtual T Value<T>(object key)
         {
-            JToken token = this[key];
+            JToken? token = this[key];
 
             // null check to fix MonoTouch issue - https://github.com/dolbz/Newtonsoft.Json/commit/a24e3062846b30ee505f3271ac08862bb471b822
             return token == null ? default : Extensions.Convert<JToken, T>(token);
@@ -340,13 +346,13 @@ namespace Newtonsoft.Json.Linq
         /// Get the first child token of this token.
         /// </summary>
         /// <value>A <see cref="JToken"/> containing the first child token of the <see cref="JToken"/>.</value>
-        public virtual JToken First => throw new InvalidOperationException("Cannot access child value on {0}.".FormatWith(CultureInfo.InvariantCulture, GetType()));
+        public virtual JToken? First => throw new InvalidOperationException("Cannot access child value on {0}.".FormatWith(CultureInfo.InvariantCulture, GetType()));
 
         /// <summary>
         /// Get the last child token of this token.
         /// </summary>
         /// <value>A <see cref="JToken"/> containing the last child token of the <see cref="JToken"/>.</value>
-        public virtual JToken Last => throw new InvalidOperationException("Cannot access child value on {0}.".FormatWith(CultureInfo.InvariantCulture, GetType()));
+        public virtual JToken? Last => throw new InvalidOperationException("Cannot access child value on {0}.".FormatWith(CultureInfo.InvariantCulture, GetType()));
 
         /// <summary>
         /// Returns a collection of the child tokens of this token, in document order.
@@ -441,7 +447,7 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        private static JValue EnsureValue(JToken value)
+        private static JValue? EnsureValue(JToken value)
         {
             if (value == null)
             {
@@ -453,7 +459,7 @@ namespace Newtonsoft.Json.Linq
                 value = property.Value;
             }
 
-            JValue v = value as JValue;
+            JValue? v = value as JValue;
 
             return v;
         }
@@ -483,7 +489,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator bool(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, BooleanTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Boolean.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -507,7 +513,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator DateTimeOffset(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, DateTimeTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to DateTimeOffset.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -539,7 +545,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, BooleanTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Boolean.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -562,7 +568,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator long(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Int64.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -590,7 +596,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, DateTimeTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to DateTime.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -619,7 +625,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, DateTimeTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to DateTimeOffset.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -655,7 +661,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Decimal.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -683,7 +689,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Double.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -711,7 +717,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, CharTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Char.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -734,7 +740,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator int(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Int32.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -757,7 +763,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator short(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Int16.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -781,7 +787,7 @@ namespace Newtonsoft.Json.Linq
         [CLSCompliant(false)]
         public static explicit operator ushort(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to UInt16.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -805,7 +811,7 @@ namespace Newtonsoft.Json.Linq
         [CLSCompliant(false)]
         public static explicit operator char(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, CharTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Char.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -828,7 +834,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator byte(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Byte.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -852,7 +858,7 @@ namespace Newtonsoft.Json.Linq
         [CLSCompliant(false)]
         public static explicit operator sbyte(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to SByte.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -880,7 +886,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Int32.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -908,7 +914,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Int16.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -937,7 +943,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to UInt16.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -965,7 +971,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Byte.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -994,7 +1000,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to SByte.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1017,7 +1023,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator DateTime(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, DateTimeTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to DateTime.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1045,7 +1051,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Int64.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1073,7 +1079,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Single.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1096,7 +1102,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator decimal(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Decimal.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1125,7 +1131,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to UInt32.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1154,7 +1160,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to UInt64.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1177,7 +1183,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator double(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Double.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1200,7 +1206,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator float(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Single.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1221,14 +1227,14 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="value">The value.</param>
         /// <returns>The result of the conversion.</returns>
-        public static explicit operator string(JToken value)
+        public static explicit operator string?(JToken value)
         {
             if (value == null)
             {
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, StringTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to String.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1262,7 +1268,7 @@ namespace Newtonsoft.Json.Linq
         [CLSCompliant(false)]
         public static explicit operator uint(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to UInt32.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1286,7 +1292,7 @@ namespace Newtonsoft.Json.Linq
         [CLSCompliant(false)]
         public static explicit operator ulong(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, NumberTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to UInt64.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1307,14 +1313,14 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="value">The value.</param>
         /// <returns>The result of the conversion.</returns>
-        public static explicit operator byte[](JToken value)
+        public static explicit operator byte[]?(JToken value)
         {
             if (value == null)
             {
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, BytesTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to byte array.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1346,7 +1352,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator Guid(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, GuidTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to Guid.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1372,7 +1378,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, GuidTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Guid.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1398,7 +1404,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>The result of the conversion.</returns>
         public static explicit operator TimeSpan(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, TimeSpanTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to TimeSpan.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1419,7 +1425,7 @@ namespace Newtonsoft.Json.Linq
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, TimeSpanTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to TimeSpan.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1438,14 +1444,14 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="value">The value.</param>
         /// <returns>The result of the conversion.</returns>
-        public static explicit operator Uri(JToken value)
+        public static explicit operator Uri?(JToken value)
         {
             if (value == null)
             {
                 return null;
             }
 
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, UriTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to Uri.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1462,18 +1468,18 @@ namespace Newtonsoft.Json.Linq
 #if HAVE_BIG_INTEGER
         private static BigInteger ToBigInteger(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, BigIntegerTypes, false))
             {
                 throw new ArgumentException("Can not convert {0} to BigInteger.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
             }
 
-            return ConvertUtils.ToBigInteger(v.Value);
+            return ConvertUtils.ToBigInteger(v.Value!);
         }
 
         private static BigInteger? ToBigIntegerNullable(JToken value)
         {
-            JValue v = EnsureValue(value);
+            JValue? v = EnsureValue(value);
             if (v == null || !ValidateToken(v, BigIntegerTypes, true))
             {
                 throw new ArgumentException("Can not convert {0} to BigInteger.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
@@ -1867,7 +1873,7 @@ namespace Newtonsoft.Json.Linq
 
         internal abstract int GetDeepHashCode();
 
-        IJEnumerable<JToken> IJEnumerable<JToken>.this[object key] => this[key];
+        IJEnumerable<JToken> IJEnumerable<JToken>.this[object key] => this[key]!;
 
         /// <summary>
         /// Creates a <see cref="JsonReader"/> for this token.
@@ -1887,7 +1893,7 @@ namespace Newtonsoft.Json.Linq
             using (JTokenWriter jsonWriter = new JTokenWriter())
             {
                 jsonSerializer.Serialize(jsonWriter, o);
-                token = jsonWriter.Token;
+                token = jsonWriter.Token!;
             }
 
             return token;
@@ -1919,9 +1925,12 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <typeparam name="T">The object type that the token will be deserialized to.</typeparam>
         /// <returns>The new object created from the JSON value.</returns>
+        [return: MaybeNull]
         public T ToObject<T>()
         {
+#pragma warning disable CS8601 // Possible null reference assignment.
             return (T)ToObject(typeof(T));
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -1929,7 +1938,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="objectType">The object type that the token will be deserialized to.</param>
         /// <returns>The new object created from the JSON value.</returns>
-        public object ToObject(Type objectType)
+        public object? ToObject(Type objectType)
         {
             if (JsonConvert.DefaultSettings == null)
             {
@@ -1947,7 +1956,7 @@ namespace Newtonsoft.Json.Linq
                         catch (Exception ex)
                         {
                             Type enumType = objectType.IsEnum() ? objectType : Nullable.GetUnderlyingType(objectType);
-                            throw new ArgumentException("Could not convert '{0}' to {1}.".FormatWith(CultureInfo.InvariantCulture, (string)this, enumType.Name), ex);
+                            throw new ArgumentException("Could not convert '{0}' to {1}.".FormatWith(CultureInfo.InvariantCulture, (string?)this, enumType.Name), ex);
                         }
                     }
 
@@ -2023,13 +2032,13 @@ namespace Newtonsoft.Json.Linq
                         return (DateTimeOffset)this;
 #endif
                     case PrimitiveTypeCode.String:
-                        return (string)this;
+                        return (string?)this;
                     case PrimitiveTypeCode.GuidNullable:
                         return (Guid?)this;
                     case PrimitiveTypeCode.Guid:
                         return (Guid)this;
                     case PrimitiveTypeCode.Uri:
-                        return (Uri)this;
+                        return (Uri?)this;
                     case PrimitiveTypeCode.TimeSpanNullable:
                         return (TimeSpan?)this;
                     case PrimitiveTypeCode.TimeSpan:
@@ -2052,9 +2061,12 @@ namespace Newtonsoft.Json.Linq
         /// <typeparam name="T">The object type that the token will be deserialized to.</typeparam>
         /// <param name="jsonSerializer">The <see cref="JsonSerializer"/> that will be used when creating the object.</param>
         /// <returns>The new object created from the JSON value.</returns>
+        [return: MaybeNull]
         public T ToObject<T>(JsonSerializer jsonSerializer)
         {
+#pragma warning disable CS8601 // Possible null reference assignment.
             return (T)ToObject(typeof(T), jsonSerializer);
+#pragma warning restore CS8601 // Possible null reference assignment.
         }
 
         /// <summary>
@@ -2063,7 +2075,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="objectType">The object type that the token will be deserialized to.</param>
         /// <param name="jsonSerializer">The <see cref="JsonSerializer"/> that will be used when creating the object.</param>
         /// <returns>The new object created from the JSON value.</returns>
-        public object ToObject(Type objectType, JsonSerializer jsonSerializer)
+        public object? ToObject(Type objectType, JsonSerializer jsonSerializer)
         {
             ValidationUtils.ArgumentNotNull(jsonSerializer, nameof(jsonSerializer));
 
@@ -2098,7 +2110,7 @@ namespace Newtonsoft.Json.Linq
         /// that were read from the reader. The runtime type of the token is determined
         /// by the token type of the first token encountered in the reader.
         /// </returns>
-        public static JToken ReadFrom(JsonReader reader, JsonLoadSettings settings)
+        public static JToken ReadFrom(JsonReader reader, JsonLoadSettings? settings)
         {
             ValidationUtils.ArgumentNotNull(reader, nameof(reader));
 
@@ -2123,7 +2135,7 @@ namespace Newtonsoft.Json.Linq
                 throw JsonReaderException.Create(reader, "Error reading JToken from JsonReader.");
             }
 
-            IJsonLineInfo lineInfo = reader as IJsonLineInfo;
+            IJsonLineInfo? lineInfo = reader as IJsonLineInfo;
 
             switch (reader.TokenType)
             {
@@ -2145,7 +2157,7 @@ namespace Newtonsoft.Json.Linq
                     v.SetLineInfo(lineInfo, settings);
                     return v;
                 case JsonToken.Comment:
-                    v = JValue.CreateComment(reader.Value.ToString());
+                    v = JValue.CreateComment(reader.Value!.ToString());
                     v.SetLineInfo(lineInfo, settings);
                     return v;
                 case JsonToken.Null:
@@ -2178,7 +2190,7 @@ namespace Newtonsoft.Json.Linq
         /// <param name="settings">The <see cref="JsonLoadSettings"/> used to load the JSON.
         /// If this is <c>null</c>, default load settings will be used.</param>
         /// <returns>A <see cref="JToken"/> populated from the string that contains JSON.</returns>
-        public static JToken Parse(string json, JsonLoadSettings settings)
+        public static JToken Parse(string json, JsonLoadSettings? settings)
         {
             using (JsonReader reader = new JsonTextReader(new StringReader(json)))
             {
@@ -2189,7 +2201,6 @@ namespace Newtonsoft.Json.Linq
                     // Any content encountered here other than a comment will throw in the reader.
                 }
 
-
                 return t;
             }
         }
@@ -2205,7 +2216,7 @@ namespace Newtonsoft.Json.Linq
         /// that were read from the reader. The runtime type of the token is determined
         /// by the token type of the first token encountered in the reader.
         /// </returns>
-        public static JToken Load(JsonReader reader, JsonLoadSettings settings)
+        public static JToken Load(JsonReader reader, JsonLoadSettings? settings)
         {
             return ReadFrom(reader, settings);
         }
@@ -2224,7 +2235,7 @@ namespace Newtonsoft.Json.Linq
             return Load(reader, null);
         }
 
-        internal void SetLineInfo(IJsonLineInfo lineInfo, JsonLoadSettings settings)
+        internal void SetLineInfo(IJsonLineInfo? lineInfo, JsonLoadSettings? settings)
         {
             if (settings != null && settings.LineInfoHandling != LineInfoHandling.Load)
             {
@@ -2265,7 +2276,7 @@ namespace Newtonsoft.Json.Linq
         {
             get
             {
-                LineInfoAnnotation annotation = Annotation<LineInfoAnnotation>();
+                LineInfoAnnotation? annotation = Annotation<LineInfoAnnotation>();
                 if (annotation != null)
                 {
                     return annotation.LineNumber;
@@ -2279,7 +2290,7 @@ namespace Newtonsoft.Json.Linq
         {
             get
             {
-                LineInfoAnnotation annotation = Annotation<LineInfoAnnotation>();
+                LineInfoAnnotation? annotation = Annotation<LineInfoAnnotation>();
                 if (annotation != null)
                 {
                     return annotation.LinePosition;
@@ -2296,7 +2307,7 @@ namespace Newtonsoft.Json.Linq
         /// A <see cref="String"/> that contains a JPath expression.
         /// </param>
         /// <returns>A <see cref="JToken"/>, or <c>null</c>.</returns>
-        public JToken SelectToken(string path)
+        public JToken? SelectToken(string path)
         {
             return SelectToken(path, false);
         }
@@ -2309,11 +2320,11 @@ namespace Newtonsoft.Json.Linq
         /// </param>
         /// <param name="errorWhenNoMatch">A flag to indicate whether an error should be thrown if no tokens are found when evaluating part of the expression.</param>
         /// <returns>A <see cref="JToken"/>.</returns>
-        public JToken SelectToken(string path, bool errorWhenNoMatch)
+        public JToken? SelectToken(string path, bool errorWhenNoMatch)
         {
             JPath p = new JPath(path);
 
-            JToken token = null;
+            JToken? token = null;
             foreach (JToken t in p.Evaluate(this, this, errorWhenNoMatch))
             {
                 if (token != null)
@@ -2438,7 +2449,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <typeparam name="T">The type of the annotation to retrieve.</typeparam>
         /// <returns>The first annotation object that matches the specified type, or <c>null</c> if no annotation is of the specified type.</returns>
-        public T Annotation<T>() where T : class
+        public T? Annotation<T>() where T : class
         {
             if (_annotations != null)
             {
@@ -2469,7 +2480,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="type">The <see cref="Type"/> of the annotation to retrieve.</param>
         /// <returns>The first annotation object that matches the specified type, or <c>null</c> if no annotation is of the specified type.</returns>
-        public object Annotation(Type type)
+        public object? Annotation(Type type)
         {
             if (type == null)
             {
@@ -2595,7 +2606,7 @@ namespace Newtonsoft.Json.Linq
         {
             if (_annotations != null)
             {
-                if (!(_annotations is object[] annotations))
+                if (!(_annotations is object?[] annotations))
                 {
                     if (_annotations is T)
                     {
@@ -2608,7 +2619,7 @@ namespace Newtonsoft.Json.Linq
                     int keepCount = 0;
                     while (index < annotations.Length)
                     {
-                        object obj2 = annotations[index];
+                        object? obj2 = annotations[index];
                         if (obj2 == null)
                         {
                             break;
@@ -2650,7 +2661,7 @@ namespace Newtonsoft.Json.Linq
 
             if (_annotations != null)
             {
-                if (!(_annotations is object[] annotations))
+                if (!(_annotations is object?[] annotations))
                 {
                     if (type.IsInstanceOfType(_annotations))
                     {
@@ -2663,7 +2674,7 @@ namespace Newtonsoft.Json.Linq
                     int keepCount = 0;
                     while (index < annotations.Length)
                     {
-                        object o = annotations[index];
+                        object? o = annotations[index];
                         if (o == null)
                         {
                             break;

Src/Newtonsoft.Json/Linq/JTokenReader.cs

@@ -34,14 +34,14 @@ namespace Newtonsoft.Json.Linq
     public class JTokenReader : JsonReader, IJsonLineInfo
     {
         private readonly JToken _root;
-        private string _initialPath;
-        private JToken _parent;
-        private JToken _current;
+        private string? _initialPath;
+        private JToken? _parent;
+        private JToken? _current;
 
         /// <summary>
         /// Gets the <see cref="JToken"/> at the reader's current position.
         /// </summary>
-        public JToken CurrentToken => _current;
+        public JToken? CurrentToken => _current;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JTokenReader"/> class.
@@ -102,8 +102,8 @@ namespace Newtonsoft.Json.Linq
                 return ReadToEnd();
             }
 
-            JToken next = t.Next;
-            if ((next == null || next == t) || t == t.Parent.Last)
+            JToken? next = t.Next;
+            if ((next == null || next == t) || t == t.Parent!.Last)
             {
                 if (t.Parent == null)
                 {
@@ -146,7 +146,7 @@ namespace Newtonsoft.Json.Linq
 
         private bool ReadInto(JContainer c)
         {
-            JToken firstChild = c.First;
+            JToken? firstChild = c.First;
             if (firstChild == null)
             {
                 return SetEnd(c);
@@ -215,7 +215,7 @@ namespace Newtonsoft.Json.Linq
                     break;
                 case JTokenType.Date:
                     {
-                        object v = ((JValue)token).Value;
+                        object? v = ((JValue)token).Value;
                         if (v is DateTime dt)
                         {
                             v = DateTimeUtils.EnsureDateTime(dt, DateTimeZoneHandling);
@@ -235,7 +235,7 @@ namespace Newtonsoft.Json.Linq
                     break;
                 case JTokenType.Uri:
                     {
-                        object v = ((JValue)token).Value;
+                        object? v = ((JValue)token).Value;
                         SetToken(JsonToken.String, v is Uri uri ? uri.OriginalString : SafeToString(v));
                         break;
                     }
@@ -247,7 +247,7 @@ namespace Newtonsoft.Json.Linq
             }
         }
 
-        private string SafeToString(object value)
+        private string? SafeToString(object? value)
         {
             return value?.ToString();
         }
@@ -259,7 +259,7 @@ namespace Newtonsoft.Json.Linq
                 return false;
             }
 
-            IJsonLineInfo info = _current;
+            IJsonLineInfo? info = _current;
             return (info != null && info.HasLineInfo());
         }
 
@@ -272,7 +272,7 @@ namespace Newtonsoft.Json.Linq
                     return 0;
                 }
 
-                IJsonLineInfo info = _current;
+                IJsonLineInfo? info = _current;
                 if (info != null)
                 {
                     return info.LineNumber;
@@ -291,7 +291,7 @@ namespace Newtonsoft.Json.Linq
                     return 0;
                 }
 
-                IJsonLineInfo info = _current;
+                IJsonLineInfo? info = _current;
                 if (info != null)
                 {
                     return info.LinePosition;

Src/Newtonsoft.Json/Linq/JTokenWriter.cs

@@ -24,6 +24,7 @@
 #endregion
 
 using System;
+using System.Diagnostics;
 using System.Globalization;
 #if HAVE_BIG_INTEGER
 using System.Numerics;
@@ -37,22 +38,22 @@ namespace Newtonsoft.Json.Linq
     /// </summary>
     public partial class JTokenWriter : JsonWriter
     {
-        private JContainer _token;
-        private JContainer _parent;
+        private JContainer? _token;
+        private JContainer? _parent;
         // used when writer is writing single value and the value has no containing parent
-        private JValue _value;
-        private JToken _current;
+        private JValue? _value;
+        private JToken? _current;
 
         /// <summary>
         /// Gets the <see cref="JToken"/> at the writer's current position.
         /// </summary>
-        public JToken CurrentToken => _current;
+        public JToken? CurrentToken => _current;
 
         /// <summary>
         /// Gets the token being written.
         /// </summary>
         /// <value>The token being written.</value>
-        public JToken Token
+        public JToken? Token
         {
             get
             {
@@ -131,7 +132,7 @@ namespace Newtonsoft.Json.Linq
         private void RemoveParent()
         {
             _current = _parent;
-            _parent = _parent.Parent;
+            _parent = _parent!.Parent;
 
             if (_parent != null && _parent.Type == JTokenType.Property)
             {
@@ -186,12 +187,12 @@ namespace Newtonsoft.Json.Linq
             base.WritePropertyName(name);
         }
 
-        private void AddValue(object value, JsonToken token)
+        private void AddValue(object? value, JsonToken token)
         {
             AddValue(new JValue(value), token);
         }
 
-        internal void AddValue(JValue value, JsonToken token)
+        internal void AddValue(JValue? value, JsonToken token)
         {
             if (_parent != null)
             {
@@ -216,7 +217,7 @@ namespace Newtonsoft.Json.Linq
         /// An error will be raised if the value cannot be written as a single JSON token.
         /// </summary>
         /// <param name="value">The <see cref="Object"/> value to write.</param>
-        public override void WriteValue(object value)
+        public override void WriteValue(object? value)
         {
 #if HAVE_BIG_INTEGER
             if (value is BigInteger)
@@ -253,7 +254,7 @@ namespace Newtonsoft.Json.Linq
         /// Writes raw JSON.
         /// </summary>
         /// <param name="json">The raw JSON to write.</param>
-        public override void WriteRaw(string json)
+        public override void WriteRaw(string? json)
         {
             base.WriteRaw(json);
             AddValue(new JRaw(json), JsonToken.Raw);
@@ -263,7 +264,7 @@ namespace Newtonsoft.Json.Linq
         /// Writes a comment <c>/*...*/</c> containing the specified text.
         /// </summary>
         /// <param name="text">Text to place inside the comment.</param>
-        public override void WriteComment(string text)
+        public override void WriteComment(string? text)
         {
             base.WriteComment(text);
             AddValue(JValue.CreateComment(text), JsonToken.Comment);
@@ -273,7 +274,7 @@ namespace Newtonsoft.Json.Linq
         /// Writes a <see cref="String"/> value.
         /// </summary>
         /// <param name="value">The <see cref="String"/> value to write.</param>
-        public override void WriteValue(string value)
+        public override void WriteValue(string? value)
         {
             base.WriteValue(value);
             AddValue(value, JsonToken.String);
@@ -446,7 +447,7 @@ namespace Newtonsoft.Json.Linq
         /// Writes a <see cref="Byte"/>[] value.
         /// </summary>
         /// <param name="value">The <see cref="Byte"/>[] value to write.</param>
-        public override void WriteValue(byte[] value)
+        public override void WriteValue(byte[]? value)
         {
             base.WriteValue(value);
             AddValue(value, JsonToken.Bytes);
@@ -476,7 +477,7 @@ namespace Newtonsoft.Json.Linq
         /// Writes a <see cref="Uri"/> value.
         /// </summary>
         /// <param name="value">The <see cref="Uri"/> value to write.</param>
-        public override void WriteValue(Uri value)
+        public override void WriteValue(Uri? value)
         {
             base.WriteValue(value);
             AddValue(value, JsonToken.String);
@@ -496,7 +497,7 @@ namespace Newtonsoft.Json.Linq
                     }
                 }
 
-                JToken value = tokenReader.CurrentToken.CloneToken();
+                JToken value = tokenReader.CurrentToken!.CloneToken();
 
                 if (_parent != null)
                 {

Src/Newtonsoft.Json/Linq/JValue.Async.cs

@@ -49,7 +49,7 @@ namespace Newtonsoft.Json.Linq
         {
             if (converters != null && converters.Length > 0 && _value != null)
             {
-                JsonConverter matchingConverter = JsonSerializer.GetMatchingConverter(converters, _value.GetType());
+                JsonConverter? matchingConverter = JsonSerializer.GetMatchingConverter(converters, _value.GetType());
                 if (matchingConverter != null && matchingConverter.CanWrite)
                 {
                     // TODO: Call WriteJsonAsync when it exists.
@@ -121,13 +121,13 @@ namespace Newtonsoft.Json.Linq
 
                     return writer.WriteValueAsync(Convert.ToDateTime(_value, CultureInfo.InvariantCulture), cancellationToken);
                 case JTokenType.Bytes:
-                    return writer.WriteValueAsync((byte[])_value, cancellationToken);
+                    return writer.WriteValueAsync((byte[]?)_value, cancellationToken);
                 case JTokenType.Guid:
                     return writer.WriteValueAsync(_value != null ? (Guid?)_value : null, cancellationToken);
                 case JTokenType.TimeSpan:
                     return writer.WriteValueAsync(_value != null ? (TimeSpan?)_value : null, cancellationToken);
                 case JTokenType.Uri:
-                    return writer.WriteValueAsync((Uri)_value, cancellationToken);
+                    return writer.WriteValueAsync((Uri?)_value, cancellationToken);
             }
 
             throw MiscellaneousUtils.CreateArgumentOutOfRangeException(nameof(Type), _valueType, "Unexpected token type.");

Src/Newtonsoft.Json/Linq/JValue.cs

@@ -28,6 +28,8 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using Newtonsoft.Json.Utilities;
 using System.Globalization;
+using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 #if HAVE_DYNAMIC
 using System.Dynamic;
 using System.Linq.Expressions;
@@ -47,9 +49,9 @@ namespace Newtonsoft.Json.Linq
 #endif
     {
         private JTokenType _valueType;
-        private object _value;
+        private object? _value;
 
-        internal JValue(object value, JTokenType type)
+        internal JValue(object? value, JTokenType type)
         {
             _value = value;
             _valueType = type;
@@ -188,7 +190,7 @@ namespace Newtonsoft.Json.Linq
         /// Initializes a new instance of the <see cref="JValue"/> class with the given value.
         /// </summary>
         /// <param name="value">The value.</param>
-        public JValue(object value)
+        public JValue(object? value)
             : this(value, GetValueType(null, value))
         {
         }
@@ -241,7 +243,7 @@ namespace Newtonsoft.Json.Linq
         }
 #endif
 
-        internal static int Compare(JTokenType valueType, object objA, object objB)
+        internal static int Compare(JTokenType valueType, object? objA, object? objB)
         {
             if (objA == objB)
             {
@@ -267,8 +269,8 @@ namespace Newtonsoft.Json.Linq
                     }
                     if (objB is BigInteger integerB)
                     {
-                        return -CompareBigInteger(integerB, objA);
-                    }
+                            return -CompareBigInteger(integerB, objA);
+                        }
 #endif
                     if (objA is ulong || objB is ulong || objA is decimal || objB is decimal)
                     {
@@ -300,7 +302,7 @@ namespace Newtonsoft.Json.Linq
                         return Convert.ToDecimal(objA, CultureInfo.InvariantCulture).CompareTo(Convert.ToDecimal(objB, CultureInfo.InvariantCulture));
                     }
                     return CompareFloat(objA, objB);
-                }
+                    }
                 case JTokenType.Comment:
                 case JTokenType.String:
                 case JTokenType.Raw:
@@ -353,10 +355,10 @@ namespace Newtonsoft.Json.Linq
                         throw new ArgumentException("Object must be of type byte[].");
                     }
 
-                    byte[] bytesA = objA as byte[];
+                    byte[]? bytesA = objA as byte[];
                     Debug.Assert(bytesA != null);
 
-                    return MiscellaneousUtils.ByteArrayCompare(bytesA, bytesB);
+                    return MiscellaneousUtils.ByteArrayCompare(bytesA!, bytesB);
                 case JTokenType.Guid:
                     if (!(objB is Guid))
                     {
@@ -368,7 +370,7 @@ namespace Newtonsoft.Json.Linq
 
                     return guid1.CompareTo(guid2);
                 case JTokenType.Uri:
-                    Uri uri2 = objB as Uri;
+                    Uri? uri2 = objB as Uri;
                     if (uri2 == null)
                     {
                         throw new ArgumentException("Object must be of type Uri.");
@@ -407,7 +409,7 @@ namespace Newtonsoft.Json.Linq
         }
 
 #if HAVE_EXPRESSIONS
-        private static bool Operation(ExpressionType operation, object objA, object objB, out object result)
+        private static bool Operation(ExpressionType operation, object? objA, object? objB, out object? result)
         {
             if (objA is string || objB is string)
             {
@@ -564,7 +566,7 @@ namespace Newtonsoft.Json.Linq
         /// </summary>
         /// <param name="value">The value.</param>
         /// <returns>A <see cref="JValue"/> comment with the given value.</returns>
-        public static JValue CreateComment(string value)
+        public static JValue CreateComment(string? value)
         {
             return new JValue(value, JTokenType.Comment);
         }
@@ -597,7 +599,7 @@ namespace Newtonsoft.Json.Linq
             return new JValue(null, JTokenType.Undefined);
         }
 
-        private static JTokenType GetValueType(JTokenType? current, object value)
+        private static JTokenType GetValueType(JTokenType? current, object? value)
         {
             if (value == null)
             {
@@ -694,13 +696,13 @@ namespace Newtonsoft.Json.Linq
         /// Gets or sets the underlying token value.
         /// </summary>
         /// <value>The underlying token value.</value>
-        public object Value
+        public object? Value
         {
             get => _value;
             set
             {
-                Type currentType = _value?.GetType();
-                Type newType = value?.GetType();
+                Type? currentType = _value?.GetType();
+                Type? newType = value?.GetType();
 
                 if (currentType != newType)
                 {
@@ -720,7 +722,7 @@ namespace Newtonsoft.Json.Linq
         {
             if (converters != null && converters.Length > 0 && _value != null)
             {
-                JsonConverter matchingConverter = JsonSerializer.GetMatchingConverter(converters, _value.GetType());
+                JsonConverter? matchingConverter = JsonSerializer.GetMatchingConverter(converters, _value.GetType());
                 if (matchingConverter != null && matchingConverter.CanWrite)
                 {
                     matchingConverter.WriteJson(writer, _value, JsonSerializer.CreateDefault());
@@ -803,7 +805,7 @@ namespace Newtonsoft.Json.Linq
                     }
                     return;
                 case JTokenType.Bytes:
-                    writer.WriteValue((byte[])_value);
+                    writer.WriteValue((byte[]?)_value);
                     return;
                 case JTokenType.Guid:
                     writer.WriteValue((_value != null) ? (Guid?)_value : null);
@@ -812,7 +814,7 @@ namespace Newtonsoft.Json.Linq
                     writer.WriteValue((_value != null) ? (TimeSpan?)_value : null);
                     return;
                 case JTokenType.Uri:
-                    writer.WriteValue((Uri)_value);
+                    writer.WriteValue((Uri?)_value);
                     return;
             }
 
@@ -839,7 +841,7 @@ namespace Newtonsoft.Json.Linq
         /// <c>true</c> if the current object is equal to the <paramref name="other"/> parameter; otherwise, <c>false</c>.
         /// </returns>
         /// <param name="other">An object to compare with this object.</param>
-        public bool Equals(JValue other)
+        public bool Equals([AllowNull] JValue other)
         {
             if (other == null)
             {
@@ -858,7 +860,12 @@ namespace Newtonsoft.Json.Linq
         /// </returns>
         public override bool Equals(object obj)
         {
-            return Equals(obj as JValue);
+            if (obj is JValue v)
+            {
+                return Equals(v);
+            }
+
+            return false;
         }
 
         /// <summary>
@@ -925,7 +932,7 @@ namespace Newtonsoft.Json.Linq
         /// <returns>
         /// A <see cref="String"/> that represents this instance.
         /// </returns>
-        public string ToString(string format, IFormatProvider formatProvider)
+        public string ToString(string? format, IFormatProvider formatProvider)
         {
             if (_value == null)
             {
@@ -957,7 +964,7 @@ namespace Newtonsoft.Json.Linq
 
         private class JValueDynamicProxy : DynamicProxy<JValue>
         {
-            public override bool TryConvert(JValue instance, ConvertBinder binder, out object result)
+            public override bool TryConvert(JValue instance, ConvertBinder binder, [NotNullWhen(true)]out object? result)
             {
                 if (binder.Type == typeof(JValue) || binder.Type == typeof(JToken))
                 {
@@ -965,7 +972,7 @@ namespace Newtonsoft.Json.Linq
                     return true;
                 }
 
-                object value = instance.Value;
+                object? value = instance.Value;
 
                 if (value == null)
                 {
@@ -977,9 +984,9 @@ namespace Newtonsoft.Json.Linq
                 return true;
             }
 
-            public override bool TryBinaryOperation(JValue instance, BinaryOperationBinder binder, object arg, out object result)
+            public override bool TryBinaryOperation(JValue instance, BinaryOperationBinder binder, object arg, [NotNullWhen(true)]out object? result)
             {
-                object compareValue = arg is JValue value ? value.Value : arg;
+                object? compareValue = arg is JValue value ? value.Value : arg;
 
                 switch (binder.Operation)
                 {
@@ -1031,7 +1038,7 @@ namespace Newtonsoft.Json.Linq
             }
 
             JTokenType comparisonType;
-            object otherValue;
+            object? otherValue;
             if (obj is JValue value)
             {
                 otherValue = value.Value;
@@ -1166,7 +1173,7 @@ namespace Newtonsoft.Json.Linq
             return (DateTime)this;
         }
 
-        object IConvertible.ToType(Type conversionType, IFormatProvider provider)
+        object? IConvertible.ToType(Type conversionType, IFormatProvider provider)
         {
             return ToObject(conversionType);
         }

Src/Newtonsoft.Json/Linq/JsonPath/ArrayIndexFilter.cs

@@ -14,7 +14,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
             {
                 if (Index != null)
                 {
-                    JToken v = GetTokenIndex(t, errorWhenNoMatch, Index.GetValueOrDefault());
+                    JToken? v = GetTokenIndex(t, errorWhenNoMatch, Index.GetValueOrDefault());
 
                     if (v != null)
                     {

Src/Newtonsoft.Json/Linq/JsonPath/ArrayMultipleIndexFilter.cs

@@ -4,7 +4,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class ArrayMultipleIndexFilter : PathFilter
     {
-        public List<int> Indexes { get; set; }
+        internal List<int> Indexes;
+
+        public ArrayMultipleIndexFilter(List<int> indexes)
+        {
+            Indexes = indexes;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {
@@ -12,7 +17,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
             {
                 foreach (int i in Indexes)
                 {
-                    JToken v = GetTokenIndex(t, errorWhenNoMatch, i);
+                    JToken? v = GetTokenIndex(t, errorWhenNoMatch, i);
 
                     if (v != null)
                     {

Src/Newtonsoft.Json/Linq/JsonPath/FieldFilter.cs

@@ -6,7 +6,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class FieldFilter : PathFilter
     {
-        public string Name { get; set; }
+        internal string? Name;
+
+        public FieldFilter(string? name)
+        {
+            Name = name;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {
@@ -16,7 +21,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
                 {
                     if (Name != null)
                     {
-                        JToken v = o[Name];
+                        JToken? v = o[Name];
 
                         if (v != null)
                         {
@@ -29,9 +34,9 @@ namespace Newtonsoft.Json.Linq.JsonPath
                     }
                     else
                     {
-                        foreach (KeyValuePair<string, JToken> p in o)
+                        foreach (KeyValuePair<string, JToken?> p in o)
                         {
-                            yield return p.Value;
+                            yield return p.Value!;
                         }
                     }
                 }

Src/Newtonsoft.Json/Linq/JsonPath/FieldMultipleFilter.cs

@@ -11,7 +11,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class FieldMultipleFilter : PathFilter
     {
-        public List<string> Names { get; set; }
+        internal List<string> Names;
+
+        public FieldMultipleFilter(List<string> names)
+        {
+            Names = names;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {
@@ -21,7 +26,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
                 {
                     foreach (string name in Names)
                     {
-                        JToken v = o[name];
+                        JToken? v = o[name];
 
                         if (v != null)
                         {

Src/Newtonsoft.Json/Linq/JsonPath/JPath.cs

@@ -107,7 +107,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
                     case '(':
                         if (_currentIndex > currentPartStartIndex)
                         {
-                            string member = _expression.Substring(currentPartStartIndex, _currentIndex - currentPartStartIndex);
+                            string? member = _expression.Substring(currentPartStartIndex, _currentIndex - currentPartStartIndex);
                             if (member == "*")
                             {
                                 member = null;
@@ -136,7 +136,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
                     case '.':
                         if (_currentIndex > currentPartStartIndex)
                         {
-                            string member = _expression.Substring(currentPartStartIndex, _currentIndex - currentPartStartIndex);
+                            string? member = _expression.Substring(currentPartStartIndex, _currentIndex - currentPartStartIndex);
                             if (member == "*")
                             {
                                 member = null;
@@ -177,7 +177,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
             if (_currentIndex > currentPartStartIndex)
             {
-                string member = _expression.Substring(currentPartStartIndex, _currentIndex - currentPartStartIndex).TrimEnd();
+                string? member = _expression.Substring(currentPartStartIndex, _currentIndex - currentPartStartIndex).TrimEnd();
                 if (member == "*")
                 {
                     member = null;
@@ -196,9 +196,9 @@ namespace Newtonsoft.Json.Linq.JsonPath
             return atPathEnd;
         }
 
-        private static PathFilter CreatePathFilter(string member, bool scan)
+        private static PathFilter CreatePathFilter(string? member, bool scan)
         {
-            PathFilter filter = (scan) ? (PathFilter)new ScanFilter {Name = member} : new FieldFilter {Name = member};
+            PathFilter filter = (scan) ? (PathFilter)new ScanFilter(member) : new FieldFilter(member);
             return filter;
         }
 
@@ -230,7 +230,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
         {
             int start = _currentIndex;
             int? end = null;
-            List<int> indexes = null;
+            List<int>? indexes = null;
             int colonCount = 0;
             int? startIndex = null;
             int? endIndex = null;
@@ -262,7 +262,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
                         int index = Convert.ToInt32(indexer, CultureInfo.InvariantCulture);
 
                         indexes.Add(index);
-                        return new ArrayMultipleIndexFilter { Indexes = indexes };
+                        return new ArrayMultipleIndexFilter(indexes);
                     }
                     else if (colonCount > 0)
                     {
@@ -421,26 +421,19 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
             if (!scan)
             {
-                return new QueryFilter
-                {
-                    Expression = expression
-                };
+                return new QueryFilter(expression);
             }
             else
             {
-                return new QueryScanFilter
-                {
-                    Expression = expression
-                };
+                return new QueryScanFilter(expression);
             }
         }
 
-        private bool TryParseExpression(out List<PathFilter> expressionPath)
+        private bool TryParseExpression(out List<PathFilter>? expressionPath)
         {
             if (_expression[_currentIndex] == '$')
             {
-                expressionPath = new List<PathFilter>();
-                expressionPath.Add(RootFilter.Instance);
+                expressionPath = new List<PathFilter> { RootFilter.Instance };
             }
             else if (_expression[_currentIndex] == '@')
             {
@@ -454,7 +447,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
             _currentIndex++;
 
-            if (ParsePath(expressionPath, _currentIndex, true))
+            if (ParsePath(expressionPath!, _currentIndex, true))
             {
                 throw new JsonException("Path ended with open query.");
             }
@@ -471,12 +464,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
         {
             EatWhitespace();
 
-            if (TryParseExpression(out var expressionPath))
+            if (TryParseExpression(out List<PathFilter>? expressionPath))
             {
                 EatWhitespace();
                 EnsureLength("Path ended with open query.");
 
-                return expressionPath;
+                return expressionPath!;
             }
 
             if (TryParseValue(out var value))
@@ -492,13 +485,13 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
         private QueryExpression ParseExpression()
         {
-            QueryExpression rootExpression = null;
-            CompositeExpression parentExpression = null;
+            QueryExpression? rootExpression = null;
+            CompositeExpression? parentExpression = null;
 
             while (_currentIndex < _expression.Length)
             {
                 object left = ParseSide();
-                object right = null;
+                object? right = null;
 
                 QueryOperator op;
                 if (_expression[_currentIndex] == ')'
@@ -514,19 +507,14 @@ namespace Newtonsoft.Json.Linq.JsonPath
                     right = ParseSide();
                 }
 
-                BooleanQueryExpression booleanExpression = new BooleanQueryExpression
-                {
-                    Left = left,
-                    Operator = op,
-                    Right = right
-                };
+                BooleanQueryExpression booleanExpression = new BooleanQueryExpression(op, left, right);
 
                 if (_expression[_currentIndex] == ')')
                 {
                     if (parentExpression != null)
                     {
                         parentExpression.Expressions.Add(booleanExpression);
-                        return rootExpression;
+                        return rootExpression!;
                     }
 
                     return booleanExpression;
@@ -540,7 +528,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
                     if (parentExpression == null || parentExpression.Operator != QueryOperator.And)
                     {
-                        CompositeExpression andExpression = new CompositeExpression { Operator = QueryOperator.And };
+                        CompositeExpression andExpression = new CompositeExpression(QueryOperator.And);
 
                         parentExpression?.Expressions.Add(andExpression);
 
@@ -563,7 +551,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
                     if (parentExpression == null || parentExpression.Operator != QueryOperator.Or)
                     {
-                        CompositeExpression orExpression = new CompositeExpression { Operator = QueryOperator.Or };
+                        CompositeExpression orExpression = new CompositeExpression(QueryOperator.Or);
 
                         parentExpression?.Expressions.Add(orExpression);
 
@@ -582,7 +570,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
             throw new JsonException("Path ended with open query.");
         }
 
-        private bool TryParseValue(out object value)
+        private bool TryParseValue(out object? value)
         {
             char currentChar = _expression[_currentIndex];
             if (currentChar == '\'')
@@ -763,9 +751,9 @@ namespace Newtonsoft.Json.Linq.JsonPath
         private bool Match(string s)
         {
             int currentPosition = _currentIndex;
-            foreach (char c in s)
+            for (int i = 0; i < s.Length; i++)
             {
-                if (currentPosition < _expression.Length && _expression[currentPosition] == c)
+                if (currentPosition < _expression.Length && _expression[currentPosition] == s[i])
                 {
                     currentPosition++;
                 }
@@ -832,7 +820,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
         private PathFilter ParseQuotedField(char indexerCloseChar, bool scan)
         {
-            List<string> fields = null;
+            List<string>? fields = null;
 
             while (_currentIndex < _expression.Length)
             {
@@ -847,8 +835,8 @@ namespace Newtonsoft.Json.Linq.JsonPath
                     {
                         fields.Add(field);
                         return (scan)
-                            ? (PathFilter)new ScanMultipleFilter { Names = fields }
-                            : (PathFilter)new FieldMultipleFilter { Names = fields };
+                            ? (PathFilter)new ScanMultipleFilter(fields)
+                            : (PathFilter)new FieldMultipleFilter(fields);
                     }
                     else
                     {

Src/Newtonsoft.Json/Linq/JsonPath/PathFilter.cs

@@ -8,9 +8,8 @@ namespace Newtonsoft.Json.Linq.JsonPath
     {
         public abstract IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch);
 
-        protected static JToken GetTokenIndex(JToken t, bool errorWhenNoMatch, int index)
+        protected static JToken? GetTokenIndex(JToken t, bool errorWhenNoMatch, int index)
         {
-
             if (t is JArray a)
             {
                 if (a.Count <= index)
@@ -50,7 +49,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
             }
         }
 
-        protected static JToken GetNextScanValue(JToken originalParent, JToken container, JToken value)
+        protected static JToken? GetNextScanValue(JToken originalParent, JToken? container, JToken? value)
         {
             // step into container's values
             if (container != null && container.HasValues)
@@ -60,7 +59,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
             else
             {
                 // finished container, move to parent
-                while (value != null && value != originalParent && value == value.Parent.Last)
+                while (value != null && value != originalParent && value == value.Parent!.Last)
                 {
                     value = value.Parent;
                 }

Src/Newtonsoft.Json/Linq/JsonPath/QueryExpression.cs

@@ -32,7 +32,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
     internal abstract class QueryExpression
     {
-        public QueryOperator Operator { get; set; }
+        internal QueryOperator Operator;
+
+        public QueryExpression(QueryOperator @operator)
+        {
+            Operator = @operator;
+        }
 
         public abstract bool IsMatch(JToken root, JToken t);
     }
@@ -41,7 +46,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
     {
         public List<QueryExpression> Expressions { get; set; }
 
-        public CompositeExpression()
+        public CompositeExpression(QueryOperator @operator) : base(@operator)
         {
             Expressions = new List<QueryExpression>();
         }
@@ -76,10 +81,16 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
     internal class BooleanQueryExpression : QueryExpression
     {
-        public object Left { get; set; }
-        public object Right { get; set; }
+        public readonly object Left;
+        public readonly object? Right;
 
-        private IEnumerable<JToken> GetResult(JToken root, JToken t, object o)
+        public BooleanQueryExpression(QueryOperator @operator, object left, object? right) : base(@operator)
+        {
+            Left = left;
+            Right = right;
+        }
+
+        private IEnumerable<JToken> GetResult(JToken root, JToken t, object? o)
         {
             if (o is JToken resultToken)
             {
@@ -211,13 +222,13 @@ namespace Newtonsoft.Json.Linq.JsonPath
                 return false;
             }
 
-            string regexText = (string)pattern.Value;
+            string regexText = (string)pattern.Value!;
             int patternOptionDelimiterIndex = regexText.LastIndexOf('/');
 
             string patternText = regexText.Substring(1, patternOptionDelimiterIndex - 1);
             string optionsText = regexText.Substring(patternOptionDelimiterIndex + 1);
 
-            return Regex.IsMatch((string)input.Value, patternText, MiscellaneousUtils.GetRegexOptions(optionsText));
+            return Regex.IsMatch((string)input.Value!, patternText, MiscellaneousUtils.GetRegexOptions(optionsText));
         }
 
         internal static bool EqualsWithStringCoercion(JValue value, JValue queryValue)
@@ -240,7 +251,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
                 return false;
             }
 
-            string queryValueString = (string)queryValue.Value;
+            string queryValueString = (string)queryValue.Value!;
 
             string currentValueString;
 
@@ -258,21 +269,21 @@ namespace Newtonsoft.Json.Linq.JsonPath
                         else
 #endif
                         {
-                            DateTimeUtils.WriteDateTimeString(writer, (DateTime)value.Value, DateFormatHandling.IsoDateFormat, null, CultureInfo.InvariantCulture);
+                            DateTimeUtils.WriteDateTimeString(writer, (DateTime)value.Value!, DateFormatHandling.IsoDateFormat, null, CultureInfo.InvariantCulture);
                         }
 
                         currentValueString = writer.ToString();
                     }
                     break;
                 case JTokenType.Bytes:
-                    currentValueString = Convert.ToBase64String((byte[])value.Value);
+                    currentValueString = Convert.ToBase64String((byte[])value.Value!);
                     break;
                 case JTokenType.Guid:
                 case JTokenType.TimeSpan:
-                    currentValueString = value.Value.ToString();
+                    currentValueString = value.Value!.ToString();
                     break;
                 case JTokenType.Uri:
-                    currentValueString = ((Uri)value.Value).OriginalString;
+                    currentValueString = ((Uri)value.Value!).OriginalString;
                     break;
                 default:
                     return false;

Src/Newtonsoft.Json/Linq/JsonPath/QueryFilter.cs

@@ -5,7 +5,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class QueryFilter : PathFilter
     {
-        public QueryExpression Expression { get; set; }
+        internal QueryExpression Expression;
+
+        public QueryFilter(QueryExpression expression)
+        {
+            Expression = expression;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {

Src/Newtonsoft.Json/Linq/JsonPath/QueryScanFilter.cs

@@ -5,7 +5,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class QueryScanFilter : PathFilter
     {
-        public QueryExpression Expression { get; set; }
+        internal QueryExpression Expression;
+
+        public QueryScanFilter(QueryExpression expression)
+        {
+            Expression = expression;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {

Src/Newtonsoft.Json/Linq/JsonPath/ScanFilter.cs

@@ -4,7 +4,12 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class ScanFilter : PathFilter
     {
-        public string Name { get; set; }
+        internal string? Name;
+
+        public ScanFilter(string? name)
+        {
+            Name = name;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {
@@ -15,11 +20,11 @@ namespace Newtonsoft.Json.Linq.JsonPath
                     yield return c;
                 }
 
-                JToken value = c;
+                JToken? value = c;
 
                 while (true)
                 {
-                    JContainer container = value as JContainer;
+                    JContainer? container = value as JContainer;
 
                     value = GetNextScanValue(c, container, value);
                     if (value == null)

Src/Newtonsoft.Json/Linq/JsonPath/ScanMultipleFilter.cs

@@ -4,17 +4,22 @@ namespace Newtonsoft.Json.Linq.JsonPath
 {
     internal class ScanMultipleFilter : PathFilter
     {
-        public List<string> Names { get; set; }
+        private List<string> _names;
+
+        public ScanMultipleFilter(List<string> names)
+        {
+            _names = names;
+        }
 
         public override IEnumerable<JToken> ExecuteFilter(JToken root, IEnumerable<JToken> current, bool errorWhenNoMatch)
         {
             foreach (JToken c in current)
             {
-                JToken value = c;
+                JToken? value = c;
 
                 while (true)
                 {
-                    JContainer container = value as JContainer;
+                    JContainer? container = value as JContainer;
 
                     value = GetNextScanValue(c, container, value);
                     if (value == null)
@@ -24,7 +29,7 @@ namespace Newtonsoft.Json.Linq.JsonPath
 
                     if (value is JProperty property)
                     {
-                        foreach (string name in Names)
+                        foreach (string name in _names)
                         {
                             if (property.Name == name)
                             {
@@ -32,7 +37,6 @@ namespace Newtonsoft.Json.Linq.JsonPath
                             }
                         }
                     }
-
                 }
             }
         }

Src/Newtonsoft.Json/Newtonsoft.Json.csproj

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <TargetFrameworks Condition="'$(LibraryFrameworks)'==''">net45;net40;net35;net20;netstandard1.0;netstandard1.3;netstandard2.0;portable-net45+win8+wpa81+wp8;portable-net40+win8+wpa81+wp8+sl5</TargetFrameworks>
     <TargetFrameworks Condition="'$(LibraryFrameworks)'!=''">$(LibraryFrameworks)</TargetFrameworks>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>8.0</LangVersion>
     <!-- version numbers will be updated by build -->
     <AssemblyVersion>11.0.0.0</AssemblyVersion>
     <FileVersion>11.0.1</FileVersion>
@@ -25,6 +25,7 @@
     <RootNamespace>Newtonsoft.Json</RootNamespace>
     <AssemblyName>Newtonsoft.Json</AssemblyName>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
+    <Nullable>enable</Nullable>
     <MinClientVersion>2.12</MinClientVersion>
     <IncludeSymbols>true</IncludeSymbols>
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
@@ -37,7 +38,9 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="2.9.2" PrivateAssets="All" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-18618-05" PrivateAssets="All" />
-  </ItemGroup> 
+    <!-- Compiler to support nullable in non-preview VS2019 -->
+    <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="3.3.0-beta1-final" PrivateAssets="All" />
+  </ItemGroup>
   <PropertyGroup Condition="'$(TargetFramework)'=='net45'">
     <AssemblyTitle>Json.NET</AssemblyTitle>
     <DefineConstants>HAVE_ADO_NET;HAVE_APP_DOMAIN;HAVE_ASYNC;HAVE_BIG_INTEGER;HAVE_BINARY_FORMATTER;HAVE_BINARY_SERIALIZATION;HAVE_BINARY_EXCEPTION_SERIALIZATION;HAVE_CAS;HAVE_CHAR_TO_LOWER_WITH_CULTURE;HAVE_CHAR_TO_STRING_WITH_CULTURE;HAVE_COM_ATTRIBUTES;HAVE_COMPONENT_MODEL;HAVE_CONCURRENT_COLLECTIONS;HAVE_COVARIANT_GENERICS;HAVE_DATA_CONTRACTS;HAVE_DATE_TIME_OFFSET;HAVE_DB_NULL_TYPE_CODE;HAVE_DYNAMIC;HAVE_EMPTY_TYPES;HAVE_ENTITY_FRAMEWORK;HAVE_EXPRESSIONS;HAVE_FAST_REVERSE;HAVE_FSHARP_TYPES;HAVE_FULL_REFLECTION;HAVE_GUID_TRY_PARSE;HAVE_HASH_SET;HAVE_ICLONEABLE;HAVE_ICONVERTIBLE;HAVE_IGNORE_DATA_MEMBER_ATTRIBUTE;HAVE_INOTIFY_COLLECTION_CHANGED;HAVE_INOTIFY_PROPERTY_CHANGING;HAVE_ISET;HAVE_LINQ;HAVE_MEMORY_BARRIER;HAVE_METHOD_IMPL_ATTRIBUTE;HAVE_NON_SERIALIZED_ATTRIBUTE;HAVE_READ_ONLY_COLLECTIONS;HAVE_REFLECTION_EMIT;HAVE_SECURITY_SAFE_CRITICAL_ATTRIBUTE;HAVE_SERIALIZATION_BINDER_BIND_TO_NAME;HAVE_STREAM_READER_WRITER_CLOSE;HAVE_STRING_JOIN_WITH_ENUMERABLE;HAVE_TIME_SPAN_PARSE_WITH_CULTURE;HAVE_TIME_SPAN_TO_STRING_WITH_CULTURE;HAVE_TIME_ZONE_INFO;HAVE_TRACE_WRITER;HAVE_TYPE_DESCRIPTOR;HAVE_UNICODE_SURROGATE_DETECTION;HAVE_VARIANT_TYPE_PARAMETERS;HAVE_VERSION_TRY_PARSE;HAVE_XLINQ;HAVE_XML_DOCUMENT;HAVE_XML_DOCUMENT_TYPE;HAVE_CONCURRENT_DICTIONARY;$(AdditionalConstants)</DefineConstants>

Src/Newtonsoft.Json/Schema/Extensions.cs

@@ -28,6 +28,8 @@ using System.Collections.Generic;
 using Newtonsoft.Json.Linq;
 using Newtonsoft.Json.Utilities;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     /// <summary>

Src/Newtonsoft.Json/Schema/JsonSchema.cs

@@ -30,6 +30,8 @@ using Newtonsoft.Json.Linq;
 using Newtonsoft.Json.Utilities;
 using System.Globalization;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     /// <summary>

Src/Newtonsoft.Json/Schema/JsonSchemaBuilder.cs

@@ -35,6 +35,8 @@ using System.Globalization;
 using Newtonsoft.Json.Utilities;
 using Newtonsoft.Json.Linq;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]

Src/Newtonsoft.Json/Schema/JsonSchemaConstants.cs

@@ -26,6 +26,8 @@
 using System;
 using System.Collections.Generic;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]

Src/Newtonsoft.Json/Schema/JsonSchemaException.cs

@@ -26,6 +26,8 @@
 using System;
 using System.Runtime.Serialization;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     /// <summary>

Src/Newtonsoft.Json/Schema/JsonSchemaGenerator.cs

@@ -37,6 +37,8 @@ using System.Linq;
 
 #endif
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     /// <summary>

Src/Newtonsoft.Json/Schema/JsonSchemaModel.cs

@@ -28,6 +28,8 @@ using System.Collections.Generic;
 using Newtonsoft.Json.Linq;
 using Newtonsoft.Json.Utilities;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]

Src/Newtonsoft.Json/Schema/JsonSchemaModelBuilder.cs

@@ -32,6 +32,8 @@ using System.Linq;
 
 #endif
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]

Src/Newtonsoft.Json/Schema/JsonSchemaNode.cs

@@ -33,6 +33,8 @@ using System.Linq;
 
 #endif
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]

Src/Newtonsoft.Json/Schema/JsonSchemaNodeCollection.cs

@@ -26,6 +26,8 @@
 using System;
 using System.Collections.ObjectModel;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]

Src/Newtonsoft.Json/Schema/JsonSchemaResolver.cs

@@ -32,6 +32,8 @@ using System.Linq;
 
 #endif
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     /// <summary>

Src/Newtonsoft.Json/Schema/JsonSchemaType.cs

@@ -25,6 +25,8 @@
 
 using System;
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     /// <summary>

Src/Newtonsoft.Json/Schema/JsonSchemaWriter.cs

@@ -35,6 +35,8 @@ using System.Linq;
 
 #endif
 
+#nullable disable
+
 namespace Newtonsoft.Json.Schema
 {
     [Obsolete("JSON Schema validation has been moved to its own package. See https://www.newtonsoft.com/jsonschema for more details.")]