MVC3.0 修改商品提交时出错。该如何处理

MVC3.0 修改商品提交时出错。
刚开始学，照着书上弄的，不知道为什么出错，大家帮帮我。

控制器敌源码：
  public ActionResult ProductList()
  {
  List<Product> products = (from p in eshop.Product
  orderby p.ProductID descending
  select p).ToList();

  return View(products);
  }

  public ActionResult AddProduct()
  {
  return View();
  }

  public ActionResult SaveProduct()
  {
  Product product = new Product();
  product.Name = Request.Form["Name"];
  product.Description = Request.Form["Description"];
  product.Price = Convert.ToDecimal(Request.Form["Price"]);
  string savePath = HttpContext.Request.MapPath(@"/content/images/");
  product.ImageUrl = SaveFile(HttpContext.Request.Files[0], savePath);

  eshop.AddToProduct(product);
  eshop.SaveChanges();

  return RedirectToAction("ProductList");
  }

  public ActionResult EditProduct(int id)
  {
  Product product = (from p in eshop.Product
  where p.ProductID == id
  select p).First();

  if (Request.RequestType == "POST")
  {
  product.Name = Request.Form["Name"];
  product.Description = Request.Form["Description"];
  product.Price = Convert.ToDecimal(Request.Form["Price"]);
  string savePath = HttpContext.Request.MapPath(@"/content/images/");
  product.ImageUrl = SaveFile(HttpContext.Request.Files[0], savePath);

  eshop.AddToProduct(product);
  eshop.SaveChanges();

  return RedirectToAction("ProductList");
  }

  return View(product);
  }


视图（View）:
  <h2>修改商品信息</h2>

  <form action="../EditProduct/<%=ViewData.Model.ProductID %>" method="post" enctype="multipart/form-data">
   
  <p>商品名称：<%=Html.TextBox("Name") %></p>
  <p>商品描述：<%=Html.TextBox("Description") %></p>
  <p>商品价格：<%=Html.TextBox("Price") %></p>
  <p>选择图片：<input type="file" name="ImageUrl" /></p>

  <p>
  <input type="submit" value="修 改" />
  <input type="button" value="取 消" />
  </p>
   
  </form>


新加产商品时就可以，就是修改不行，帮忙看看为什么？

------解决方案--------------------
MVC我只懂皮毛，看你edit的函数中，有 eshop.AddToProduct(product); 在我看来，这个不应该出现。


------解决方案--------------------
if (Request.RequestType == "POST")
{
product.Name = Request.Form["Name"];
product.Description = Request.Form["Description"];
product.Price = Convert.ToDecimal(Request.Form["Price"]);
string savePath = HttpContext.Request.MapPath(@"/content/images/");
product.ImageUrl = SaveFile(HttpContext.Request.Files[0], savePath);