如何在c＃datagrid的列标题中显示/隐藏图像？

Question

我有一个数据网格，其中有许多列，其中2个可排序，日期和时间。量。只要单击任一列，我的代码就可以重新排序行（asc-to-desc，desc-to-asc）。但是，我在每个列标题中都有3个图标 - 向上箭头，向下箭头和显示两者的默认图标。

如何在页面加载时显示默认图标，向上箭头仅显示订单何时上升，下降时显示下降？刚才我显示了所有3个图标，任何修复此问题的尝试都会引发错误。

以下是datagrid的相关部分（显示Amount列）：

     <asp:datagrid id="dgCurrentRequests" runat="server" Width="100%" Font-Size="XX-Small" Font-Names="Verdana"
                            AutoGenerateColumns="False" DataKeyField="RequestID" AllowSorting="true" OnSortCommand="OnSort">
                            <FooterStyle Font-Bold="True"></FooterStyle>
                            <HeaderStyle Font-Size="XX-Small" Font-Names="Verdana" Font-Bold="True" ForeColor="White" BackColor="#006699"></HeaderStyle>
                            <Columns>
                                <asp:BoundColumn DataField="BranchName" HeaderText="Branch"></asp:BoundColumn>
                                <asp:BoundColumn DataField="AccountantName" HeaderText="Requested By"></asp:BoundColumn>
                                <asp:BoundColumn DataField="Supplier" HeaderText="Pay To"></asp:BoundColumn>


<asp:TemplateColumn SortExpression="DisplayAmount">
                <HeaderTemplate>
                                <asp:LinkButton id="DisplayAmount" runat="server" Text="Amount"  
                                        CommandName="Sort" CommandArgument="DisplayAmount"></asp:LinkButton>
                                <img runat="server" id="AmountAscending" src="..\images\asc.png" />
                                <img runat="server" id="AmountDefault" src="..\images\default.png" />
                                <img runat="server" id="AmountDescending" src="..\images\desc.png" />
                </HeaderTemplate>
                <itemtemplate>
                                <asp:Label id="DisplayAmount" runat="server" Text='<%# DataBinder.Eval(Container.DataItem,"DisplayAmount") %>' ></asp:Label>
                </itemtemplate>
</asp:TemplateColumn>

以下是重新排序网格的代码隐藏：

 public void FillCurrentRequests()
        {
            Accountant theAccountant=(Accountant)Session["Accountant"];
            ArrayList requests = new ArrayList();
            requests = theAccountant.GetExternalRoylineRequests(dbConnString, "Pending", (string)ViewState["sortColumn"], (string)ViewState["sortDirection"], 100);

            dgCurrentRequests.DataSource = requests;
            dgCurrentRequests.DataBind();

        }

        public void OnSort(Object sender, DataGridSortCommandEventArgs e)
        {
            if (((string) ViewState["sortColumn"]).Equals(e.SortExpression))
            {
                if (((string) ViewState["sortDirection"]) == "ASC")
                {
                    ViewState["sortDirection"] = "DESC";
                }
                else
                {
                    ViewState["sortDirection"] = "ASC";
                }
            }
            else
            {
                ViewState["sortDirection"] = "ASC";
            }

            ViewState["sortColumn"] = e.SortExpression;

            FillCurrentRequests();
        }




I have attempted to declare the images then use them in a method to set the default icon, but it throws a SystemNullException:

        protected Image AmountAscending;
        protected Image AmountDefault;
        protected Image AmountDescending;
        protected Image DateRequestedAscending;
        protected Image DateRequestedDefault;
        protected Image DateRequestedDescending;

//

        public void ResetSortOrder()
        {
            AmountAscending.Visible = false;
            ...
        }

就好像应用程序看不到cs文件实体'AmountAscending'等是aspx文件中这些图像的id。

请帮忙！如何在aspx.cs页面中使用这些图像？