MVC model type conditional binding

Question

I have an action:

[HttpPost]
public ActionResult(Animal a)
{

}

I'd like a to be a Rabbit or Dog depending on the incoming form data. Is there a simple way to achieve this?
Thank you

Answer 1

In order to get this to work, you are looking at setting up your Action to accept a dynamic parameter, that a ModelBinder will convert to the appropriate type, either a Rabbit or Dog:

[HttpPost]
public ActionResult([ModelBinder(typeof(AnimalBinder))] dynamic a)
{

}

Since the Action doesn't know what the object is that it is getting, it needs a way of knowing what to convert that object to. You will need two things to achieve this. First, you have to embed in your View, EditorTemplate, whatever, what the model that you are binding to is:

@Html.Hidden("ModelType", Model.GetType())

Second, the model binder that will create an object of the appropriate type, based on the ModelType field you specified above:

public class AnimalBinder : DefaultModelBinder
    {
        protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
        {
            var typeValue = bindingContext.ValueProvider.GetValue("ModelType");
            var type = Type.GetType((string)typeValue.ConvertTo(typeof(string)), true);
            if (!typeof(Animal).IsAssignableFrom(type))
            {
                throw new InvalidOperationException("Bad Type");
            }
            var model = Activator.CreateInstance(type);
            bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, type);
            return model;
        }
    }

Once this is all in place, if you inspect the dynamic a object that is passed into the Action, you will see that it is of type Rabbit or Dog based on what the page model was.