Make sure that

"valueField": null

is parsed appropriately, i.e. that it remembers that the field is set.

csharp/src/Google.Protobuf.Test/JsonParserTest.cs

@@ -124,7 +124,9 @@ namespace Google.Protobuf
         [Test]
         public void SingularWrappers_ExplicitNulls()
         {
-            var message = new TestWellKnownTypes();
+            // When we parse the "valueField": null part, we remember it... basically, it's one case
+            // where explicit default values don't fully roundtrip.
+            var message = new TestWellKnownTypes { ValueField = Value.ForNull() };
             var json = new JsonFormatter(new JsonFormatter.Settings(true)).Format(message);
             var parsed = JsonParser.Default.Parse<TestWellKnownTypes>(json);
             Assert.AreEqual(message, parsed);
@@ -150,6 +152,14 @@ namespace Google.Protobuf
             Assert.AreEqual(expected, parsed);
         }
 
+        [Test]
+        public void ExplicitNullValue()
+        {
+            string json = "{\"valueField\": null}";
+            var message = JsonParser.Default.Parse<TestWellKnownTypes>(json);
+            Assert.AreEqual(new TestWellKnownTypes { ValueField = Value.ForNull() }, message);
+        }
+
         [Test]
         public void BytesWrapper_Standalone()
         {

csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs

@@ -31,7 +31,7 @@ namespace Google.Protobuf.TestProtos {
             "L3Byb3RvYnVmL3NvdXJjZV9jb250ZXh0LnByb3RvGhxnb29nbGUvcHJvdG9i",
             "dWYvc3RydWN0LnByb3RvGh9nb29nbGUvcHJvdG9idWYvdGltZXN0YW1wLnBy",
             "b3RvGhpnb29nbGUvcHJvdG9idWYvdHlwZS5wcm90bxoeZ29vZ2xlL3Byb3Rv",
-            "YnVmL3dyYXBwZXJzLnByb3RvIpEHChJUZXN0V2VsbEtub3duVHlwZXMSJwoJ",
+            "YnVmL3dyYXBwZXJzLnByb3RvIr4HChJUZXN0V2VsbEtub3duVHlwZXMSJwoJ",
             "YW55X2ZpZWxkGAEgASgLMhQuZ29vZ2xlLnByb3RvYnVmLkFueRInCglhcGlf",
             "ZmllbGQYAiABKAsyFC5nb29nbGUucHJvdG9idWYuQXBpEjEKDmR1cmF0aW9u",
             "X2ZpZWxkGAMgASgLMhkuZ29vZ2xlLnByb3RvYnVmLkR1cmF0aW9uEisKC2Vt",
@@ -51,7 +51,8 @@ namespace Google.Protobuf.TestProtos {
             "cm90b2J1Zi5VSW50MzJWYWx1ZRIuCgpib29sX2ZpZWxkGBAgASgLMhouZ29v",
             "Z2xlLnByb3RvYnVmLkJvb2xWYWx1ZRIyCgxzdHJpbmdfZmllbGQYESABKAsy",
             "HC5nb29nbGUucHJvdG9idWYuU3RyaW5nVmFsdWUSMAoLYnl0ZXNfZmllbGQY",
-            "EiABKAsyGy5nb29nbGUucHJvdG9idWYuQnl0ZXNWYWx1ZSKVBwoWUmVwZWF0",
+            "EiABKAsyGy5nb29nbGUucHJvdG9idWYuQnl0ZXNWYWx1ZRIrCgt2YWx1ZV9m",
+            "aWVsZBgTIAEoCzIWLmdvb2dsZS5wcm90b2J1Zi5WYWx1ZSKVBwoWUmVwZWF0",
             "ZWRXZWxsS25vd25UeXBlcxInCglhbnlfZmllbGQYASADKAsyFC5nb29nbGUu",
             "cHJvdG9idWYuQW55EicKCWFwaV9maWVsZBgCIAMoCzIULmdvb2dsZS5wcm90",
             "b2J1Zi5BcGkSMQoOZHVyYXRpb25fZmllbGQYAyADKAsyGS5nb29nbGUucHJv",
@@ -162,7 +163,7 @@ namespace Google.Protobuf.TestProtos {
       descriptor = pbr::FileDescriptor.FromGeneratedCode(descriptorData,
           new pbr::FileDescriptor[] { global::Google.Protobuf.WellKnownTypes.AnyReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.ApiReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.DurationReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.EmptyReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.FieldMaskReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.SourceContextReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.StructReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.TimestampReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.TypeReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.WrappersReflection.Descriptor, },
           new pbr::GeneratedCodeInfo(null, new pbr::GeneratedCodeInfo[] {
-            new pbr::GeneratedCodeInfo(typeof(global::Google.Protobuf.TestProtos.TestWellKnownTypes), global::Google.Protobuf.TestProtos.TestWellKnownTypes.Parser, new[]{ "AnyField", "ApiField", "DurationField", "EmptyField", "FieldMaskField", "SourceContextField", "StructField", "TimestampField", "TypeField", "DoubleField", "FloatField", "Int64Field", "Uint64Field", "Int32Field", "Uint32Field", "BoolField", "StringField", "BytesField" }, null, null, null),
+            new pbr::GeneratedCodeInfo(typeof(global::Google.Protobuf.TestProtos.TestWellKnownTypes), global::Google.Protobuf.TestProtos.TestWellKnownTypes.Parser, new[]{ "AnyField", "ApiField", "DurationField", "EmptyField", "FieldMaskField", "SourceContextField", "StructField", "TimestampField", "TypeField", "DoubleField", "FloatField", "Int64Field", "Uint64Field", "Int32Field", "Uint32Field", "BoolField", "StringField", "BytesField", "ValueField" }, null, null, null),
             new pbr::GeneratedCodeInfo(typeof(global::Google.Protobuf.TestProtos.RepeatedWellKnownTypes), global::Google.Protobuf.TestProtos.RepeatedWellKnownTypes.Parser, new[]{ "AnyField", "ApiField", "DurationField", "EmptyField", "FieldMaskField", "SourceContextField", "StructField", "TimestampField", "TypeField", "DoubleField", "FloatField", "Int64Field", "Uint64Field", "Int32Field", "Uint32Field", "BoolField", "StringField", "BytesField" }, null, null, null),
             new pbr::GeneratedCodeInfo(typeof(global::Google.Protobuf.TestProtos.OneofWellKnownTypes), global::Google.Protobuf.TestProtos.OneofWellKnownTypes.Parser, new[]{ "AnyField", "ApiField", "DurationField", "EmptyField", "FieldMaskField", "SourceContextField", "StructField", "TimestampField", "TypeField", "DoubleField", "FloatField", "Int64Field", "Uint64Field", "Int32Field", "Uint32Field", "BoolField", "StringField", "BytesField" }, new[]{ "OneofField" }, null, null),
             new pbr::GeneratedCodeInfo(typeof(global::Google.Protobuf.TestProtos.MapWellKnownTypes), global::Google.Protobuf.TestProtos.MapWellKnownTypes.Parser, new[]{ "AnyField", "ApiField", "DurationField", "EmptyField", "FieldMaskField", "SourceContextField", "StructField", "TimestampField", "TypeField", "DoubleField", "FloatField", "Int64Field", "Uint64Field", "Int32Field", "Uint32Field", "BoolField", "StringField", "BytesField" }, null, null, new pbr::GeneratedCodeInfo[] { null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, })
@@ -215,6 +216,7 @@ namespace Google.Protobuf.TestProtos {
       BoolField = other.BoolField;
       StringField = other.StringField;
       BytesField = other.BytesField;
+      ValueField = other.valueField_ != null ? other.ValueField.Clone() : null;
     }
 
     public TestWellKnownTypes Clone() {
@@ -410,6 +412,19 @@ namespace Google.Protobuf.TestProtos {
       }
     }
 
+    /// <summary>Field number for the "value_field" field.</summary>
+    public const int ValueFieldFieldNumber = 19;
+    private global::Google.Protobuf.WellKnownTypes.Value valueField_;
+    /// <summary>
+    ///  Part of struct, but useful to be able to test separately
+    /// </summary>
+    public global::Google.Protobuf.WellKnownTypes.Value ValueField {
+      get { return valueField_; }
+      set {
+        valueField_ = value;
+      }
+    }
+
     public override bool Equals(object other) {
       return Equals(other as TestWellKnownTypes);
     }
@@ -439,6 +454,7 @@ namespace Google.Protobuf.TestProtos {
       if (BoolField != other.BoolField) return false;
       if (StringField != other.StringField) return false;
       if (BytesField != other.BytesField) return false;
+      if (!object.Equals(ValueField, other.ValueField)) return false;
       return true;
     }
 
@@ -462,6 +478,7 @@ namespace Google.Protobuf.TestProtos {
       if (boolField_ != null) hash ^= BoolField.GetHashCode();
       if (stringField_ != null) hash ^= StringField.GetHashCode();
       if (bytesField_ != null) hash ^= BytesField.GetHashCode();
+      if (valueField_ != null) hash ^= ValueField.GetHashCode();
       return hash;
     }
 
@@ -533,6 +550,10 @@ namespace Google.Protobuf.TestProtos {
       if (bytesField_ != null) {
         _single_bytesField_codec.WriteTagAndValue(output, BytesField);
       }
+      if (valueField_ != null) {
+        output.WriteRawTag(154, 1);
+        output.WriteMessage(ValueField);
+      }
     }
 
     public int CalculateSize() {
@@ -591,6 +612,9 @@ namespace Google.Protobuf.TestProtos {
       if (bytesField_ != null) {
         size += _single_bytesField_codec.CalculateSizeWithTag(BytesField);
       }
+      if (valueField_ != null) {
+        size += 2 + pb::CodedOutputStream.ComputeMessageSize(ValueField);
+      }
       return size;
     }
 
@@ -697,6 +721,12 @@ namespace Google.Protobuf.TestProtos {
           BytesField = other.BytesField;
         }
       }
+      if (other.valueField_ != null) {
+        if (valueField_ == null) {
+          valueField_ = new global::Google.Protobuf.WellKnownTypes.Value();
+        }
+        ValueField.MergeFrom(other.ValueField);
+      }
     }
 
     public void MergeFrom(pb::CodedInputStream input) {
@@ -832,6 +862,13 @@ namespace Google.Protobuf.TestProtos {
             }
             break;
           }
+          case 154: {
+            if (valueField_ == null) {
+              valueField_ = new global::Google.Protobuf.WellKnownTypes.Value();
+            }
+            input.ReadMessage(valueField_);
+            break;
+          }
         }
       }
     }

csharp/src/Google.Protobuf/JsonParser.cs

@@ -215,11 +215,16 @@ namespace Google.Protobuf
             var token = tokenizer.Next();
             if (token.Type == JsonToken.TokenType.Null)
             {
+                // Clear the field if we see a null token, unless it's for a singular field of type
+                // google.protobuf.Value.
                 // Note: different from Java API, which just ignores it.
                 // TODO: Bring it more in line? Discuss...
+                if (field.IsMap || field.IsRepeated || !IsGoogleProtobufValueField(field))
+                {
                     field.Accessor.Clear(message);
                     return;
                 }
+            }
             tokenizer.PushBack(token);
 
             if (field.IsMap)
@@ -297,14 +302,22 @@ namespace Google.Protobuf
             }
         }
 
+        private static bool IsGoogleProtobufValueField(FieldDescriptor field)
+        {
+            return field.FieldType == FieldType.Message &&
+                field.MessageType.FullName == Value.Descriptor.FullName;
+        }
+
         private object ParseSingleValue(FieldDescriptor field, JsonTokenizer tokenizer)
         {
             var token = tokenizer.Next();
             if (token.Type == JsonToken.TokenType.Null)
             {
-                if (field.FieldType == FieldType.Message && field.MessageType.FullName == Value.Descriptor.FullName)
+                // TODO: In order to support dynamic messages, we should really build this up
+                // dynamically.
+                if (IsGoogleProtobufValueField(field))
                 {
-                    return new Value { NullValue = NullValue.NULL_VALUE };
+                    return Value.ForNull();
                 }
                 return null;
             }

src/google/protobuf/unittest_well_known_types.proto

@@ -39,6 +39,8 @@ message TestWellKnownTypes {
   google.protobuf.BoolValue bool_field = 16;
   google.protobuf.StringValue string_field = 17;
   google.protobuf.BytesValue bytes_field = 18;
+  // Part of struct, but useful to be able to test separately
+  google.protobuf.Value value_field = 19;
 }
 
 // A repeated field for each well-known type.