Json.NET custom serialization with JsonConverter - how to get the "default" behavior

Solution 1:

One easy way to do it is to allocate an instance of your class then use JsonSerializer.Populate(JsonReader, Object). This is the way it is done in the standard CustomCreationConverter<T>:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
    if (reader.Value != null && reader.ValueType == typeof(string))
    {
        return someSpecialDataTypeInstance;
    }
    else if (reader.TokenType == JsonToken.StartObject)
    {
        existingValue = existingValue ?? serializer.ContractResolver.ResolveContract(objectType).DefaultCreator();
        serializer.Populate(reader, existingValue);
        return existingValue;
    }
    else if (reader.TokenType == JsonToken.Null)
    {
        return null;
    }
    else
    {
        throw new JsonSerializationException();
    }
}

Limitations:

• This does not handle situations when TypeNameHandling has been enabled and a "$type" property is present specifying a polymorphic subtype.

  In this case you'll need to do some of the tricks use by JsonDerivedTypeConverer<T> in JsonConverter with Interface.

• The type to be deserialized must have a parameterless constructor accessible to Json.NET. If not, and existingValue is null, it will be necessary to construct it manually, via new DataType(arg1, arg2, ...).

• Reference preservation via PreserveReferencesHandling is not supported.

  For one approach to handle this situation see How can I choose what type to deserialize at runtime based on the structure of the json?.

Sample fiddle.