json.net - how to add property $type ONLY on root object

Question

I want to modify my json.NET serializer to add the $type property only to the objects which implements a given interface but not to any property or nested objects.

With TypeNameHandling.Auto (default)

{
  "PropertyA": 123,
  "PropertyB": "foo",
  "PropertyC": [1, 2, 3, 4]
}

With TypeNameHandling.All

{
  "$type": "JsonNetTypeNameHandling.TestEvent, jsonNetTypeNameHandling",
  "PropertyA": 123,
  "PropertyB": "foo",
  "PropertyC": {
    "$type": "System.Collections.Generic.List`1[[System.Int32, mscorlib]], mscorlib",
    "$values": [1, 2, 3, 4 ]
  }
}

What I want

{
  "$type": "JsonNetTypeNameHandling.TestEvent, jsonNetTypeNameHandling",
  "PropertyA": 123,
  "PropertyB": "foo",
  "PropertyC": [1, 2, 3, 4]
}

I am experimenting with a custom ContractResolver but I don't get it to work:

class Program
{
    static void Main(string[] args)
    {
        var serializerSettings = new JsonSerializerSettings()
        {
            TypeNameHandling = TypeNameHandling.Auto,
            TypeNameAssemblyFormat = System.Runtime.Serialization.Formatters.FormatterAssemblyStyle.Simple,
            NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore,
            ContractResolver = new EnableTypeNameHandlingAllOnlyForEvents(),
            Formatting = Formatting.Indented
        };

        var event1 = new TestEvent() { PropertyA = 123, PropertyB = "foo", PropertyC = new List<int> { 1, 2, 3, 4 } };

        string event1Serialized = JsonConvert.SerializeObject(event1, serializerSettings);

        Console.WriteLine(event1Serialized);
        Console.ReadLine();
    }
}

public interface IEvent
{
}

public class TestEvent : IEvent
{
    public int PropertyA { get; set; }
    public string PropertyB { get; set; }
    public List<int> PropertyC { get; set; }
}

public class EnableTypeNameHandlingAllOnlyForEvents : DefaultContractResolver
{
    protected override JsonObjectContract CreateObjectContract(Type objectType)
    {
        var x = base.CreateObjectContract(objectType);

        if (typeof(IEvent).IsAssignableFrom(x.UnderlyingType))
        {
            // What to do to tell json.NET to add $type to instances of this (IEvent) type???
        }

        return x;
    }
}

Answer 1

If you require the "$type" property on your root object and are OK with it appearing on nested polymorphic objects and arrays if required, use the following overload along with TypeNameHandling.Auto: JsonConvert.SerializeObject(Object, Type, JsonSerializerSettings).

From the docs:

public static string SerializeObject(
    Object value,
    Type type,
    JsonSerializerSettings settings
)

type Type: System.Type The type of the value being serialized. This parameter is used when TypeNameHandling is Auto to write out the type name if the type of the value does not match. Specifing the type is optional.

I.e., do:

var serializerSettings = new JsonSerializerSettings()
{
    TypeNameHandling = TypeNameHandling.Auto,
    TypeNameAssemblyFormat = System.Runtime.Serialization.Formatters.FormatterAssemblyStyle.Simple,
    NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore,
    Formatting = Formatting.Indented
};

var event1Serialized = JsonConvert.SerializeObject(event1, typeof(IEvent), serializerSettings);

If you require "$type" on the root object and will not accept it on nested polymorphic objects and arrays even if otherwise required, you will need to use TypeNameHandling.All along with a custom contract resolver that sets JsonContainerContract.ItemTypeNameHandling = TypeNameHandling.None:

public class SuppressItemTypeNameContractResolver : DefaultContractResolver
{
    protected override JsonContract CreateContract(Type objectType)
    {
        var contract = base.CreateContract(objectType);
        var containerContract = contract as JsonContainerContract;
        if (containerContract != null)
        {
            if (containerContract.ItemTypeNameHandling == null)
                containerContract.ItemTypeNameHandling = TypeNameHandling.None; 
        }
        return contract;
    }
}

Then use it like:

static IContractResolver suppressItemTypeNameContractResolver = new SuppressItemTypeNameContractResolver();

var serializerSettings = new JsonSerializerSettings()
{
    TypeNameHandling = TypeNameHandling.All,
    ContractResolver = suppressItemTypeNameContractResolver,
    // Other settings as required.
    TypeNameAssemblyFormat = System.Runtime.Serialization.Formatters.FormatterAssemblyStyle.Simple,
    NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore,
    Formatting = Formatting.Indented
};
var event1Serialized = JsonConvert.SerializeObject(event1, serializerSettings);

Notes:

• Be aware of this caution from the Newtonsoft docs:

  TypeNameHandling should be used with caution when your application deserializes JSON from an external source. Incoming types should be validated with a custom SerializationBinder when deserializing with a value other than None.

  For a discussion of why this may be necessary, see TypeNameHandling caution in Newtonsoft Json, How to configure Json.NET to create a vulnerable web API, and Alvaro Mu?oz & Oleksandr Mirosh's blackhat paper https://www.blackhat.com/docs/us-17/thursday/us-17-Munoz-Friday-The-13th-JSON-Attacks-wp.pdf

• You may want to statically cache the contract resolver for best performance.