什么会导致ASP.NET MVC 1.0中的模型绑定失败?

这是我的课……有什么建议吗?

public class Cart
{
    private List<CartLine> lines = new List<CartLine>();
    public IList<CartLine> Lines { get { return lines.AsReadOnly(); } }

    public void AddItem(Product product, int quantity)
    {
        var line = lines.FirstOrDefault(l => l.Product.ProductID == product.ProductID);

        if (line == null)
            lines.Add(new CartLine { Product = product, Quantity = quantity });
        else
            line.Quantity += quantity;

    }

    public decimal ComputeTotalValue() 
    {
        return lines.Sum(l => l.Product.Price * l.Quantity);
    }

    public void Clear() 
    {
        lines.Clear();
    }

    public void RemoveLine(Product product)
    {
        lines.RemoveAll(l => l.Product.ProductID == product.ProductID);
    }
}

public class CartLine
{
    public Product Product { get; set;}
    public int Quantity { get; set;}
}

在控制器中添加操作方法。

public RedirectToRouteResult AddToCart(Cart cart, int productID, string returnUrl)
        {
            Product product = productsRepository.Products.FirstOrDefault(p => p.ProductID == productID);
            cart.AddItem(product, 1);
            return RedirectToAction("Index", new { returnUrl });
        }

        public RedirectToRouteResult RemoveFromCart(Cart cart, int productID, string returnUrl)
        {
            Product product = productsRepository.Products.FirstOrDefault(p => p.ProductID == productID);
            cart.RemoveLine(product);
            return RedirectToAction("Index", new { returnUrl });
        }

        public ViewResult Index(Cart cart, string returnUrl)
        {
            ViewData["returnUrl"] = returnUrl;
            ViewData["CurrentCategory"] = "Cart";
            return View(cart);
        }

我的自定义模型活页夹:

public class CartModelBinder : IModelBinder
    {
        private const string cartSessionKey = "_cart";

        #region IModelBinder Members

        public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            if (bindingContext.Model != null)
                throw new InvalidOperationException("Cannot update instances");
            Cart cart = (Cart)controllerContext.HttpContext.Session[cartSessionKey];
            if (cart == null)
            {
                cart = new Cart();
                controllerContext.HttpContext.Session["cartSessionKey"] = cart;
            }
            return cart;
        }
        #endregion
    }