Polymorphic model binding

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.