-Fixed error when deserializing null value into ConcurrentDictionary

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

@@ -61,6 +61,20 @@ namespace Newtonsoft.Json.Tests.Serialization
     [TestFixture]
     public class JsonSerializerCollectionsTests : TestFixtureBase
     {
+#if !(NET35 || NET20 || PORTABLE || PORTABLE40) || NETSTANDARD2_0
+        [Test]
+        public void DeserializeConcurrentDictionaryWithNullValue()
+        {
+            const string key = "id";
+            
+            var jsonValue = $"{{\"{key}\":null}}";
+
+            var deserializedObject = JsonConvert.DeserializeObject<ConcurrentDictionary<string, string>>(jsonValue);
+
+            Assert.IsNull(deserializedObject[key]);
+        }
+#endif
+
 #if !(NET20 || NET35)
         [Test]
         public void SerializeConcurrentQueue()

Src/Newtonsoft.Json/Serialization/DefaultContractResolver.cs

@@ -900,7 +900,7 @@ namespace Newtonsoft.Json.Serialization
                     case "System.Collections.Concurrent.ConcurrentQueue`1":
                     case "System.Collections.Concurrent.ConcurrentStack`1":
                     case "System.Collections.Concurrent.ConcurrentBag`1":
-                    case "System.Collections.Concurrent.ConcurrentDictionary`2":
+                    case JsonTypeReflector.ConcurrentDictionaryTypeName:
                     case "System.Collections.ObjectModel.ObservableCollection`1":
                         return true;
                 }

Src/Newtonsoft.Json/Serialization/JsonDictionaryContract.cs

@@ -127,10 +127,22 @@ namespace Newtonsoft.Json.Serialization
                 {
                     CreatedType = typeof(Dictionary<,>).MakeGenericType(keyType, valueType);
                 }
+                else if (underlyingType.IsGenericType())
+                {
+                    // ConcurrentDictionary<,> + IDictionary setter + null value = error
+                    // wrap to use generic setter
+                    // https://github.com/JamesNK/Newtonsoft.Json/issues/1582
+                    Type typeDefinition = underlyingType.GetGenericTypeDefinition();
+                    if (typeDefinition.FullName == JsonTypeReflector.ConcurrentDictionaryTypeName)
+                    {
+                        ShouldCreateWrapper = true;
+                    }
+                }
 
 #if HAVE_READ_ONLY_COLLECTIONS
                 IsReadOnlyOrFixedSize = ReflectionUtils.InheritsGenericDefinition(underlyingType, typeof(ReadOnlyDictionary<,>));
 #endif
+
             }
 #if HAVE_READ_ONLY_COLLECTIONS
             else if (ReflectionUtils.ImplementsGenericDefinition(underlyingType, typeof(IReadOnlyDictionary<,>), out _genericCollectionDefinitionType))
@@ -172,7 +184,10 @@ namespace Newtonsoft.Json.Serialization
 #endif
             }
 
-            ShouldCreateWrapper = !typeof(IDictionary).IsAssignableFrom(CreatedType);
+            if (!typeof(IDictionary).IsAssignableFrom(CreatedType))
+            {
+                ShouldCreateWrapper = true;
+            }
 
             DictionaryKeyType = keyType;
             DictionaryValueType = valueType;

Src/Newtonsoft.Json/Serialization/JsonTypeReflector.cs

@@ -56,6 +56,8 @@ namespace Newtonsoft.Json.Serialization
         public const string ShouldSerializePrefix = "ShouldSerialize";
         public const string SpecifiedPostfix = "Specified";
 
+        public const string ConcurrentDictionaryTypeName = "System.Collections.Concurrent.ConcurrentDictionary`2";
+
         private static readonly ThreadSafeStore<Type, Func<object[], object>> CreatorCache = 
             new ThreadSafeStore<Type, Func<object[], object>>(GetCreator);