Polymorphic model binding

Question

This question has been asked before in earlier versions of MVC. There is also this blog entry about a way to work around the problem. I'm wondering if MVC3 has introduced anything that might help, or if there are any other options.

In a nutshell. Here's the situation. I have an abstract base model, and 2 concrete subclasses. I have a strongly typed view that renders the models with EditorForModel(). Then I have custom templates to render each concrete type.

The problem comes at post time. If I make the post action method take the base class as the parameter, then MVC can't create an abstract version of it (which i would not want anyways, i'd want it to create the actual concrete type). If I create multiple post action methods that vary only by parameter signature, then MVC complains that it's ambiguous.

So as far as I can tell, I have a few choices on how to solve this proble. I don't like any of them for various reasons, but i will list them here:

1. Create a custom model binder as Darin suggests in the first post I linked to.
2. Create a discriminator attribute as the second post I linked to suggests.
3. Post to different action methods based on type
4. ???

I don't like 1, because it is basically configuration that is hidden. Some other developer working on the code may not know about it and waste a lot of time trying to figure out why things break when changes things.

I don't like 2, because it seems kind of hacky. But, i'm leaning towards this approach.

I don't like 3, because that means violating DRY.

Any other suggestions?

Edit:

I decided to go with Darin's method, but made a slight change. I added this to my abstract model:

[HiddenInput(DisplayValue = false)]
public string ConcreteModelType { get { return this.GetType().ToString(); }}

Then a hidden automatically gets generated in my DisplayForModel(). The only thing you have to remember is that if you're not using DisplayForModel(), you'll have to add it yourself.

Answer 1

Since I obviously opt for option 1 (:-)) let me try to elaborate it a little more so that it is less breakable and avoid hardcoding concrete instances into the model binder. The idea is to pass the concrete type into a hidden field and use reflection to instantiate the concrete type.

Suppose that you have the following view models:

public abstract class BaseViewModel
{
    public int Id { get; set; }
}

public class FooViewModel : BaseViewModel
{
    public string Foo { get; set; }
}

the following controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new FooViewModel { Id = 1, Foo = "foo" };
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(BaseViewModel model)
    {
        return View(model);
    }
}

the corresponding Index view:

@model BaseViewModel
@using (Html.BeginForm())
{
    @Html.Hidden("ModelType", Model.GetType())    
    @Html.EditorForModel()
    <input type="submit" value="OK" />
}

and the ~/Views/Home/EditorTemplates/FooViewModel.cshtml editor template:

@model FooViewModel
@Html.EditorFor(x => x.Id)
@Html.EditorFor(x => x.Foo)

Now we could have the following custom model binder:

public class BaseViewModelBinder : 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(BaseViewModel).IsAssignableFrom(type))
        {
            throw new InvalidOperationException("Bad Type");
        }
        var model = Activator.CreateInstance(type);
        bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, type);
        return model;
    }
}

The actual type is inferred from the value of the ModelType hidden field. It is not hardcoded, meaning that you could add other child types later without having to ever touch this model binder.

This same technique could be easily be applied to collections of base view models.