问在MVC中绑定模型的过程中，属性上的KnownType属性？

EN

public class Food
{
   ...
}

public class BirdFood : Food
{
    public int SeedCount { get; set; }
}

public class Creature
{
    public Food Food { get; set; }
}

public class Bird : Creature
{
   ...
}

public class ViewModel
{
    public Creature Creature { get; set; }
}

    public ActionResult Index()
    {
        ViewModel viewModel = new ViewModel();

        viewModel.Creature = new Bird();

        viewModel.Creature.Food = new BirdFood() { SeedCount = 100 } ;

        return View(viewModel);
    }

<% using(Html.BeginForm("Save", "Home", FormMethod.Post))
   { %>

   <%: Html.Hidden("Creature.Food.SeedCount", (Model.Creature.Food as MvcApplication6.Controllers.BirdFood).SeedCount)  %>

   <input type="submit" />

<% } %>