Question

我正在遵循与此帖中相同的教程：

教程页面：http://www.asp.net/web-forms/tutorials/aspnet-45/getting-started-with-aspnet-45-web-forms/shopping-cart

但是我得到了一个不同的错误，我不明白为什么。当我运行应用程序时，它工作到我收到此异常消息：

“App_Web_ia5agzom.dll中发生了'System.NullReferenceException'类型的异常，但未在用户代码中处理”

并指向ShoppingCart.aspx中的以下行：

<%# String.Format("{0:c}", ((Convert.ToDouble(Item.Quantity)) *  Convert.ToDouble(Item.Product.UnitPrice))) %>

如果我删除程序运行，但不显示每个单位的价格。

我的代码如下：

ShoppingCart.aspx：

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPages/FrontEnd.Master" AutoEventWireup="true" CodeBehind="ShoppingCart.aspx.cs" Inherits="Gbl.UserPages.ShoppingCart" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="cphMain" runat="server">
    <div id="ShoppingCartTitle" runat="server" class="ContentHead"><h1>Shopping Cart</h1></div>
    <asp:GridView ID="CartList" runat="server" 
        AutoGenerateColumns="False" 
        ShowFooter="True" 
        GridLines="Vertical" 
        CellPadding="4"
        ItemType="Gbl.Models.CartItem" 
        SelectMethod="GetShoppingCartItems"
        CssClass="table table-striped table-bordered" >   
        <Columns>
        <asp:BoundField DataField="ProductId" HeaderText="ID" SortExpression="ProductId" />        
        <asp:BoundField DataField="Product.ProductName" HeaderText="Name" />        
        <asp:BoundField DataField="Product.UnitPrice" HeaderText="Price (each)" DataFormatString="{0:c}"/>     
        <asp:TemplateField   HeaderText="Quantity">            
                <ItemTemplate>
                    <asp:TextBox ID="PurchaseQuantity" Width="40" runat="server" Text="<%#: Item.Quantity %>"></asp:TextBox> 
                </ItemTemplate>        
        </asp:TemplateField>    
        <asp:TemplateField HeaderText="Item Total">            
                <ItemTemplate>
                    <%# String.Format("{0:c}", ((Convert.ToDouble(Item.Quantity)) *  Convert.ToDouble(Item.Product.UnitPrice))) %>
                </ItemTemplate>        
        </asp:TemplateField> 
        <asp:TemplateField HeaderText="Remove Item">            
                <ItemTemplate>
                    <asp:CheckBox id="Remove" runat="server"></asp:CheckBox>
                </ItemTemplate>        
        </asp:TemplateField>    
        </Columns>    
    </asp:GridView>
    <div>
        <p></p>
        <strong>
            <asp:Label ID="LabelTotalText" runat="server" Text="Order Total: "></asp:Label>
            <asp:Label ID="lblTotal" runat="server" EnableViewState="false"></asp:Label>
        </strong> 
    </div>
    <br />
</asp:Content>

代码背后：

public partial class ShoppingCart : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            using (ShoppingCartEngine usersShoppingCart = new ShoppingCartEngine())
            {
                decimal cartTotal = 0;
                cartTotal = usersShoppingCart.GetTotal();
                if (cartTotal > 0)
                {
                    // Display Total.
                    lblTotal.Text = String.Format("{0:c}", cartTotal);

                }
                else
                {
                    LabelTotalText.Text = "labeltotaltext";
                    lblTotal.Text = "labeltotal";

                    ShoppingCartTitle.InnerText = "Shopping Cart is Empty";
                }
            }
        }

        public List<CartItem> GetShoppingCartItems()
        {
            ShoppingCartEngine engine = new ShoppingCartEngine();
            return engine.GetCartItems();
        }
    }

CardItem.cs：

public class CartItem
{
    [Key]
    public string ItemId { get; set; }
    public string CartId { get; set; }
    public int Quantity { get; set; }
    public System.DateTime DateCreated { get; set; }
    public int ProductId { get; set; }
    public Product Product { get; set; }
}

最后我的堆栈跟踪：

[NullReferenceException: Object reference not set to an instance of an object.]
   ASP.userpages_shoppingcart_aspx.__DataBind__control17(Object sender, EventArgs e) in i:\gabriel\work\school\civ_eng\gral_workspaces\visualstudio2014_ASP.NET\assignment_2\Gbl\UserPages\ShoppingCart.aspx:25
   System.Web.UI.Control.OnDataBinding(EventArgs e) +92
   System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +85
   System.Web.UI.Control.DataBind() +15
   System.Web.UI.Control.DataBindChildren() +187
   System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +94
   System.Web.UI.Control.DataBind() +15
   System.Web.UI.Control.DataBindChildren() +187
   System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +94
   System.Web.UI.Control.DataBind() +15
   System.Web.UI.WebControls.GridView.CreateRow(Int32 rowIndex, Int32 dataSourceIndex, DataControlRowType rowType, DataControlRowState rowState, Boolean dataBind, Object dataItem, DataControlField[] fields, TableRowCollection rows, PagedDataSource pagedDataSource) +167
   System.Web.UI.WebControls.GridView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding) +3724
   System.Web.UI.WebControls.CompositeDataBoundControl.PerformDataBinding(IEnumerable data) +67
   System.Web.UI.WebControls.GridView.PerformDataBinding(IEnumerable data) +14
   System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data) +123
   System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) +33
   System.Web.UI.WebControls.DataBoundControl.PerformSelect() +138
   System.Web.UI.WebControls.BaseDataBoundControl.DataBind() +30
   System.Web.UI.WebControls.GridView.DataBind() +4
   System.Web.UI.WebControls.BaseDataBoundControl.EnsureDataBound() +105
   System.Web.UI.WebControls.CompositeDataBoundControl.CreateChildControls() +75
   System.Web.UI.Control.EnsureChildControls() +83
   System.Web.UI.Control.PreRenderRecursiveInternal() +42
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +974

有人可以帮忙吗？

谢谢

Answer 1

根据教程，该行应包含UnitPrice而不是Price：

<%#: String.Format("{0:c}", ((Convert.ToDouble(Item.Quantity)) *  Convert.ToDouble(Item.Product.UnitPrice)))%>

Answer 2

为了启用延迟加载，需要将Product设置为virtual：

 public virtual Product Product { get; set; }

所以实体类应如下所示：

公共类CartItem {

[Key]
public string CartItemId { get; set; }

public string CartId { get; set; }

public int ProductId { get; set; }

public int Quantity { get; set; }

public System.DateTime DateCreated { get; set; }

public virtual Product Product { get; set; }

}

无法使用网格视图的ItemTemplate显示总计

2 个答案: