MVC 2 ViewModel绑定dropdownlist 和LIST 取值问题。

以下是我的代码，请各位指教。
// view model
public class CheckoutViewModel
    {
       // Cart products summary
        public List<Cart> CartItems { get; set; }
        public List<SelectListItem> CardTypes { get; set; }
        public decimal Total { get; set;}

    }
//controller
public ActionResult Index()
        {
            //SSLSwitch.UseHTTPS();
            CheckoutViewModel checkout = new CheckoutViewModel();
            List<Cart> cart = new List<Cart>();
            cart = shoppingCart.GetCartItems(Session.SessionID).ToList(); // 从数据库中取值
            checkout.CartItems = cart;
            checkout.Total = 0;

            //Credit Card types
            checkout.CardTypes = new List<SelectListItem>();
            checkout.CardTypes.Add(new SelectListItem
            {
                Text = "Visa",
                Value = "Visa",
                Selected = true
            });
            checkout.CardTypes.Add(new SelectListItem
            {
                Text = "MasterCard",
                Value = "MasterCard",
            });
            checkout.CardTypes.Add(new SelectListItem
            {
                Text = "Amex",
                Value = "Amex"
            });
           // 因为POST表单中不能直接取到MODEL.CARDTYPE的值，只能使用TempData
            TempData["CardType"] = checkout.CartTypes;
            TempData["CartItems"] = checkout.CartItems;

            return View(checkout);
        }

[HttpPost]

        public ActionResult Index(CheckoutViewModel checkout, string submit ,string total )
        {..}

// model 只能取到Total的值，得不到CARTTYPE 和CARTITEMS 的值 --------------------编程问答-------------------- <%=Html.DropDownListFor(model => model.SendMode, ViewData["list"]as List<SelectListItem>, new { @class = "TextBox8" })%>

补充：.NET技术 ,  ASP.NET