Sometimes we’re presented objects in JSON that do not directly map to a strongly typed object. Here’s a simple example:

[
  {
    "type": "Car",
    "wheels": 4,
    "trunk": true
  },
  {
    "type": "Bicycle",
    "wheels": 2,
    "persons": 1
  }
]

The goal is to deserialize these into their respective types, Car and Bicycle, both inheriting from a shared base class Vehicle.

JsonConverter

JsonConverter allows you to customize how JSON is deserialized. Here’s an implementation that checks the type property and instantiates the appropriate object:

public class VehicleConverter : JsonConverter
{
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JToken jObject = JToken.ReadFrom(reader);
        VehicleType type = jObject["type"].ToObject<VehicleType>();

        Vehicle result = type switch
        {
            VehicleType.Car => new Car(),
            VehicleType.Bicycle => new Bicycle(),
            _ => throw new ArgumentOutOfRangeException()
        };

        serializer.Populate(jObject.CreateReader(), result);
        return result;
    }

    // WriteJson and CanConvert omitted
}

Note: We use Populate() instead of Deserialize() to avoid recursion, which would trigger our converter again and cause a stack overflow. Newtonsoft.Json will detect this and throw a JsonSerializationException with a “self referencing loop” message.

Flexibility

This method is flexible:

  • You can auto-discover applicable types from assemblies.
  • You could use a registration system to associate types.

Serialization Caveat

The example above registers the converter directly on the Vehicle type. This makes serialization problematic, since the same converter would be called again. One workaround is to register the converter via JsonSerializerSettings instead. This allows you to restrict its use to deserialization only by configuring CanConvert() accordingly.

Example Code

The full code is available here: github.com/LordMike/blog-examples/deserialize-different-types

Example

[1] Newtonsoft.Json avoids stack overflow by throwing a JsonSerializationException about self-referencing loops.