Deserialising JSON to derived types in Asp.Net Web API

You don’t need a custom model binder. Nor do you need to muck about with the request pipeline.

Take a look at this other SO: How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects?.

I used this as the basis for my own solution to the same problem.

Starting off with the JsonCreationConverter<T> referenced in that SO (slightly modified to fix issues with serialization of types in responses):

public abstract class JsonCreationConverter<T> : JsonConverter
{
    /// <summary>
    /// this is very important, otherwise serialization breaks!
    /// </summary>
    public override bool CanWrite
    {
        get
        {
            return false;
        }
    }
    /// <summary> 
    /// Create an instance of objectType, based properties in the JSON object 
    /// </summary> 
    /// <param name="objectType">type of object expected</param> 
    /// <param name="jObject">contents of JSON object that will be 
    /// deserialized</param> 
    /// <returns></returns> 
    protected abstract T Create(Type objectType, JObject jObject);

    public override bool CanConvert(Type objectType)
    {
        return typeof(T).IsAssignableFrom(objectType);
    }

    public override object ReadJson(JsonReader reader, Type objectType,
      object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;
        // Load JObject from stream 
        JObject jObject = JObject.Load(reader);

        // Create target object based on JObject 
        T target = Create(objectType, jObject);

        // Populate the object properties 
        serializer.Populate(jObject.CreateReader(), target);

        return target;
    }

    public override void WriteJson(JsonWriter writer, object value, 
      JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
} 

And now you can annotate your type with the JsonConverterAttribute, pointing Json.Net to a custom converter:

[JsonConverter(typeof(MyCustomConverter))]
public abstract class BaseClass{
  private class MyCustomConverter : JsonCreationConverter<BaseClass>
  {
     protected override BaseClass Create(Type objectType, 
       Newtonsoft.Json.Linq.JObject jObject)
     {
       //TODO: read the raw JSON object through jObject to identify the type
       //e.g. here I'm reading a 'typename' property:

       if("DerivedType".Equals(jObject.Value<string>("typename")))
       {
         return new DerivedClass();
       }
       return new DefaultClass();

       //now the base class' code will populate the returned object.
     }
  }
}

public class DerivedClass : BaseClass {
  public string DerivedProperty { get; set; }
}

public class DefaultClass : BaseClass {
  public string DefaultProperty { get; set; }
}

Now you can use the base type as a parameter:

public Result Post(BaseClass arg) {

}

And if we were to post:

{ typename: 'DerivedType', DerivedProperty: 'hello' }

Then arg would be an instance of the DerivedClass, but if we posted:

{ DefaultProperty: 'world' }

Then you’d get an instance of the DefaultClass.

EDIT – Why I prefer this method to TypeNameHandling.Auto/All

I do believe that using the TypeNameHandling.Auto/All espoused by JotaBe is not always the ideal solution. It might well be in this case – but personally I won’t do it unless:

• My API is only ever going to be used by me or my team
• I don’t care about having a dual XML-compatible endpoint

When Json.Net TypeNameHandling.Auto or All are used, your web server will start sending out type names in the format MyNamespace.MyType, MyAssemblyName.

I have said in comments that I think this is a security concern. Mention was made of this in some documentation I read from Microsoft. It’s not mentioned any more, it seems, however I still feel it’s a valid concern. I don’t ever want to expose namespace-qualified type names and assembly names to the outside world. It’s increasing my attack surface. So, yes, I can not have Object properties/parameters my API types, but who’s to say the rest of my site is completely hole-free? Who’s to say a future endpoint doesn’t expose the ability to exploit type names? Why take that chance just because it’s easier?

Also – if you are writing a ‘proper’ API, i.e. specifically for consumption by third-parties and not just for yourself, and you’re using Web API, then you’re most likely looking to leverage the JSON/XML content-type handling (as a minimum). See how far you get trying to write documentation that’s easy to consume, which refers to all your API types differently for XML and JSON formats.

By overriding how JSON.Net understands the type names, you can bring the two into line, making the choice between XML/JSON for your caller purely based on taste, rather than because the type names are easier to remember in one or the other.