在MVC WebShop中删除列表项

Question

我正在为我的班级做一个网上商店项目，但我在删除时遇到一个错误，此行中发生异常if (cart[index].Amount == 1) 它说 mscorlib.dll中发生了'System.ArgumentOutOfRangeException'类型的异常但未在用户代码中处理

private int isExisting(int id)
        {
            List<Item> cart = (List<Item>)Session["cart"];
            for (int i = 0; i < cart.Count; i++)
            {
                if (cart[i].Cl.Id == id)
                    return i;
            }
            return -1;
        }
        public ActionResult Delete(int idDelete)
        {
            int index = isExisting(idDelete);
            List<Item> cart = (List<Item>)Session["cart"];
            if (cart[index].Amount == 1) {
                cart.RemoveAt(index);
                Session["cart"] = cart;
            }else
            {
                cart[index].Amount--;
                Session["cart"] = cart;
            }

            return View("Order"); 

        }