Can I pass an interface based object to an MVC 4 WebApi POST?

Question

I want to have an API as such:

public class RelayController : ApiController
{
    // POST api/values
    public void Post([FromBody]IDataRelayPackage package         


        

Answer 1

Json.NET (the default Json serializer for ASP.NET Web API) can handle this situation. All you need to do is change the serializer settings, and set TypeNameHandling to All (or Objects). This will add a "$type"-json property to your json, containing the type name of your instance. On the other side it'll try to deserialize to this type again.

We used this for ourselves, taking an abstract base class as type.

Answer 2

You can do this fairly easily with a custom model binder. Here is what worked for me. (Using Web API 2 and JSON.Net 6)

public class JsonPolyModelBinder : IModelBinder
{
    readonly JsonSerializerSettings settings = new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Auto };

    public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
    {
        var content = actionContext.Request.Content;
        string json = content.ReadAsStringAsync().Result;
        var obj = JsonConvert.DeserializeObject(json, bindingContext.ModelType, settings);
        bindingContext.Model = obj;
        return true;
    }
}

The Web API controller looks like this. (Note: should also work for regular MVC actions -- I've done something like this for them before as well.)

public class TestController : ApiController
{
    // POST api/test
    public void Post([ModelBinder(typeof(JsonPolyModelBinder))]ICommand command)
    {
        ...
    }
}

I should also note that when you serialize the JSON, you should serialize it with the same setting, and serialize it as an interface to make the Auto kick in and include the type hint. Something like this.

    JsonSerializerSettings settings = new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Auto };
    string json = JsonConvert.SerializeObject(command, typeof(ICommand), settings);

Answer 3

The serializer needs a type that it can construct, one with an empty (Default) constructor. Since an Interface can't be constructed, serialization fails and you get a null value.

Two options:

1. Use a POCO (Plain Old Clr Object) with a default constructor
2. Implement your own binder using IActionValueBinder

Answer 4

You are trying to deserialise to an interface. The serialiser won't know what type to instantiate unless it is told.

Take a look at TypeNameHandling option Posting a collection of subclasses

Or look at creating a custom JsonConverter. Take a look at this question How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects?