Add JsonProperty.IsRequiredSpecified (#2027)

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

@@ -781,5 +781,36 @@ namespace Newtonsoft.Json.Tests.Serialization
             Assert.AreEqual(null, property5.GetIsSpecified);
             Assert.AreEqual(null, property5.SetIsSpecified);
         }
+
+        [Test]
+        public void JsonRequiredAttribute()
+        {
+            DefaultContractResolver resolver = new DefaultContractResolver();
+
+            JsonObjectContract contract = (JsonObjectContract)resolver.ResolveContract(typeof(RequiredPropertyTestClass));
+
+            var property1 = contract.Properties["Name"];
+
+            Assert.AreEqual(Required.Always, property1.Required);
+            Assert.AreEqual(true, property1.IsRequiredSpecified);
+        }
+
+        [Test]
+        public void JsonPropertyAttribute_Required()
+        {
+            DefaultContractResolver resolver = new DefaultContractResolver();
+
+            JsonObjectContract contract = (JsonObjectContract)resolver.ResolveContract(typeof(RequiredObject));
+
+            var unset = contract.Properties["UnsetProperty"];
+
+            Assert.AreEqual(Required.Default, unset.Required);
+            Assert.AreEqual(false, unset.IsRequiredSpecified);
+
+            var allowNull = contract.Properties["AllowNullProperty"];
+
+            Assert.AreEqual(Required.AllowNull, allowNull.Required);
+            Assert.AreEqual(true, allowNull.IsRequiredSpecified);
+        }
     }
 }

Src/Newtonsoft.Json/Serialization/JsonProperty.cs

@@ -201,6 +201,11 @@ namespace Newtonsoft.Json.Serialization
             set => _required = value;
         }
 
+        /// <summary>
+        /// Gets a value indicating whether <see cref="Required"/> has a value specified.
+        /// </summary>
+        public bool IsRequiredSpecified => _required != null;
+
         /// <summary>
         /// Gets or sets a value indicating whether this property preserves object references.
         /// </summary>