Question

I’m using ASP.NET MVC2 and I have the following object structure:

public class IDealer {
  string Name { get; set; }
  List<IVehicle> Vehicles { get; set; }
}

public class DealerImpl {
  public string Name { get; set; }
  public List<IVehicle> Vehicles { get; set; }
}

public interface IVehicle {
    string Type { get; }
}

public class Car : IVehicle {
    public string Type { get { return this.GetType().FullName; } }
}

public class Truck : IVehicle {
    public string Type { get { return this.GetType().FullName; } }
}

I have the following class as my ModelBinder which deserializes objects in my page requests:

public class JsonModelBinder : DefaultModelBinder {
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {
        return deserialize(controllerContext, bindingContext);
    }

    protected static bool IsJSONRequest(ControllerContext controllerContext) {
        var contentType = controllerContext.HttpContext.Request.ContentType;
        return contentType.Contains("application/json");
    }

    protected virtual object deserialize(ControllerContext controllerContext, ModelBindingContext bindingContext) {
        Type modelType = bindingContext.ModelMetadata.ModelType;

        bool isNotConcrete = bindingContext.ModelMetadata.ModelType.IsInterface || bindingContext.ModelMetadata.ModelType.IsAbstract;
        if (!IsJSONRequest(controllerContext)) {
            return base.BindModel(controllerContext, bindingContext);
        } else {
            var request = controllerContext.HttpContext.Request;
            var jsonStringData = new StreamReader(request.InputStream).ReadToEnd();
            if (isNotConcrete) {
                Dictionary<string, Object> result = JsonConvert.DeserializeObject<Dictionary<string, Object>>(jsonStringData);
                string type = result["Type"] as string;
                modelType = Type.GetType(type + ",MyCompany.Common");
            }

            return JsonConvert.DeserializeObject(jsonStringData, modelType, new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Auto });
        }       
    }
}

// ASP.NET MVC Controller
protected override void Initialize(System.Web.Routing.RequestContext requestContext) {
  base.Initialize(requestContext);
  ModelBinders.Binders.DefaultBinder = new JsonModelBinder();
}

[HttpPost]
public ActionResult addUpdateDealer(IDealer dealer) {
  // breaks before here with the error in the comment below
}

// and in the aspx page
<script>
var model = <%= JsonConvert.SerializeObject(Model, Formatting.None, new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Auto }) %>;
</script>

The problem I’m running into, is that when the code tries to deserialize the child list of IVehicles, it does not know which type of vehicle to instantiate. I put a property on IVehicle called “Type” which could be used to help determine which class to instantiate, but I’m not sure what/where/how to provide an override to perform this check.

Answer 1

Your solution is similar to what JSON.NET has built-in now, called TypeNameHandling. Here are the release notes on that.

Your JSON message will need to include a $type property, which won’t be deserialized, but will be interpreted by the deserializer as the concrete type to use.