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

Question

I have a JsonConverter for my class DataType. I would like to do some special handling when plain string used in Json as the value of a property of type DataType. In the case where the value is a "full" object, I would like to do the "normal" deserialization.

Here is my attempt

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)
    {
        DataType dataType = serializer.Deserialize<DataType>(reader);
        return dataType;
    }
    else
    {
        throw new JsonSerializationException();
    }
}

But this doesn't work, because this line: DataType dataType = serializer.Deserialize(reader); causes infinite recursion.

Could this be done somehow easily? (without the need to manually go property-by-property)

You skip some steps when you`re trying to deserialize . Take a look at here stackoverflow.com/questions/8030538/… — Andreea Dumitru
– Andreea Dumitru, Commented Feb 23, 2016 at 20:52

dbc · Accepted Answer · 2019-09-06 02:41:37Z

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.

The DefaultConvert() call throws NRE at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue). I replaced it with direct constructor call.... apart from that, thanks for your answer!
@j_maly - i'm having the same issue, but I can't figure out what the solution is based on your comment. Can you post the modifications you made to this answer?
@dbc - Thank you for posting that fiddle. It seems you are still calling DefaultCreator(). Like j_maly, I'm getting a NullReferenceException on DefaultCreator(). j_maly said "I replaced it with direct constructor call", but I couldn't interpret what he/she meant by that. I was looking for the replacement/fix to solve the NullReferenceException issue.
@SFun28 - does your type have a public parameterless constructor? If not that might be the problem. If your type only has a single constructor, and that constructor is parameterized, then DefaultCreator() won't work. Instead you'll have to construct your type manually by calling new MyType(arg1, arg2, ... ).
@dbc - my type does not have a public parameterless constructor. I do have a customized contract resolver. I also know that if my JsonConverter return false for CanConvert, that json.net is able to deserialize the type using the customized contract resolver. Does that open up any options?

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