public ActionResult  AddToCart(Cart cart, Int32 productId, String returnUrl)
        {
            Product product = repository.Products.Where(p => p.ProductID == productId)
                .FirstOrDefault();
            if(product!=null)
            {
                cart.AddItem(product,1);
            }
            SetUserNameByViewBag();
          
            return PartialView(cart);
         }

 1 namespace BookStore.Domain.Entities
 2 {
 3     [Serializable]
 4    public  class Cart
 5     {
 6        private List<CartLine> lineCollection = new List<CartLine>();
 7        public void AddItem(Product product,Int32 quantity)
 8        {
 9            CartLine line = lineCollection.Where(p => p.Product.ProductID == product.ProductID)
10                .FirstOrDefault();
11            if(line==null)
12            {
13                lineCollection.Add(new CartLine { Product = product, Quantity = quantity });
14            }
15            else
16            {
17                line.Quantity += quantity;
18            }
19        }
20        public void RemoveLine(Product product)
21        {
22            lineCollection.RemoveAll(p => product.ProductID == p.Product.ProductID);
23        }
24        public Decimal ComputerTotalValue()
25        {
26            return lineCollection.Sum(p => p.Product.Price * p.Quantity);
27        }
28        public void Clear()
29        {
30            lineCollection.Clear();
31        }
32        public IEnumerable<CartLine> Lines
33        {
34            get
35            {
36                return lineCollection;
37            }
38        }
39     }
40     [Serializable]
41    public class CartLine
42    {
43        public Product Product{get;set;}
44        public Int32 Quantity;
45    }
46 }
47 //session 的代码
48 namespace BookStore.WebUI.Binders
49 {
50     public class CartModelBinder:IModelBinder
51     {
52         private const String sessionKey = "Cart";
53         public object BindModel(ControllerContext controllerContext,
54             ModelBindingContext bingdingContext)
55         {
56             Cart cart = (Cart)controllerContext.HttpContext.Session[sessionKey];
57             if(cart==null)
58             {
59                 cart = new Cart();
60                 controllerContext.HttpContext.Session[sessionKey] = cart;
61             }
62             return cart;
63         }
64     }
65 }