前台程式碼

<asp:GridView ID="GridView1" runat="server"   
    Width="970px" BackColor="White" BorderColor="White" BorderStyle="Ridge" 
        BorderWidth="2px" CellPadding="3"
        CellSpacing="1" GridLines="None" AutoGenerateColumns="False" 
        ShowHeaderWhenEmpty="True" EnableEventValidation="false" Visible="False"
        EmptyDataText="查無資料,請用新增按鈕新增"
        DataKeyNames="ID"
        onselectedindexchanged="GridView1_SelectedIndexChanged" > 
        <Columns>
            <asp:BoundField DataField="ID" HeaderText="ID" SortExpression="ID" />
            <asp:BoundField DataField="病歷號" HeaderText="病歷號" SortExpression="病歷號" />
            <asp:BoundField DataField="病患名稱" HeaderText="病患名稱" SortExpression="病患名稱" />
            <asp:BoundField DataField="建檔日期" HeaderText="建檔日期" SortExpression="建檔日期" />
            <asp:BoundField DataField="主治醫師" HeaderText="主治醫師" SortExpression="主治醫師" />
            <asp:HyperLinkField NavigateUrl="Cancerplan-edit.aspx" Text="修改" />
            <asp:CommandField ButtonType="Button" ShowSelectButton="True" />
        </Columns>
        <FooterStyle BackColor="#C6C3C6" ForeColor="Black" />
        <HeaderStyle BackColor="#4A3C8C" Font-Bold="True" ForeColor="#E7E7FF" />
        <PagerStyle BackColor="#C6C3C6" ForeColor="Black" HorizontalAlign="Right" />
        <RowStyle BackColor="#DEDFDE" ForeColor="Black" />
        <SelectedRowStyle BackColor="#9471DE" Font-Bold="True" ForeColor="White" />
        <SortedAscendingCellStyle BackColor="#F1F1F1" />
        <SortedAscendingHeaderStyle BackColor="#594B9C" />
        <SortedDescendingCellStyle BackColor="#CAC9C9" />
        <SortedDescendingHeaderStyle BackColor="#33276A" />
    </asp:GridView>

後臺程式碼

 protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Response.Write("gv selectedIndex: " + GridView1.SelectedIndex + "<br>"); 
        Response.Write("gv selectedRow.RowIndex: " + GridView1.SelectedRow.RowIndex + "<br>"); 
        Response.Write("gv selectedValue: " + GridView1.SelectedValue + "<br>"); 
        Response.Write("gv SelectedDataKey.Value: " + GridView1.SelectedDataKey.Value + "<br>"); 
        //<asp:CommandField ShowSelectButton="True" ButtonType="Button" /> 的捉法: 
        //Response.Write("gv 第八個欄位裡，按鈕的文字: " + ((Button)GridView1.Rows[GridView1.SelectedIndex].Cells[7].Controls[0]).Text + "<br>"); 
        //<asp:TemplateField 裡手動塞 <asp:Button ID="Button1" runat="server" CausesValidation="False" CommandName="Select" Text="選取" /> 按鈕、捉 ID 的捉法: 
        Response.Write("gv 第1個欄位裡，按鈕的文字: " + ((Button)GridView1.Rows[GridView1.SelectedIndex].Cells[1].FindControl("Button")).Text + "<br>"); 
        Response.Write("gv 第2個欄位裡，欄位裡的文字: " + GridView1.Rows[GridView1.SelectedIndex].Cells[2].Text + "<br>"); 
    }

請問一下 我已經有給予DataKeyNames
為何在執行時 仍出現
System.InvalidOperationException: 必須先在 GridView 'GridView1' 上指定資料索引鍵，才能擷取選取的資料索引鍵。請使用 DataKeyNames 屬性指定資料索引鍵。