我用gridview里的DataKeyNames属性为什么传了值。

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Grades_Add.aspx.cs" Inherits="Grades_Add" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><style type="text/css">
<!--
body {
margin-left: 0px;
margin-top: 0px;
}
-->
</style>
<link href="css/style.css" rel="stylesheet" type="text/css" />
</head>
<body>
    <form id="form1" runat="server">
    <div>
      <table width="100%" border="0" cellpadding="0" cellspacing="0" bgcolor="#f0f0f0">
        <tr>
          <td width="10%" style="height: 21px">&nbsp;</td>
          <td align="right" class="font12_black" style="width: 29%; height: 21px">年级名&nbsp;</td>
          <td align="left" style="width: 20%; height: 21px"><asp:TextBox CssClass="textbox_blackborder" ID="Grade_Name" runat="server" />
            <span class="font12_black">部名&nbsp;</span></td>
          <td width="10%" style="height: 21px"><asp:DropDownList CssClass="textbox_blackborder" ID="Dep_GDList" runat="server" DataSourceID="SqlDataSource1" DataTextField="DEP_NAME" DataValueField="DEP_ID"></asp:DropDownList></td>
          <td style="height: 21px; width: 10%;">&nbsp;<asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="~/images/add.gif"
                  OnClick="ImageButton1_Click" /></td>
          <td width="10%" style="height: 21px">&nbsp;</td>
        </tr>
      </table>
    </div>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ESchoolConnectionString %>" SelectCommand="SELECT [DEP_ID], [DEP_NAME] FROM [DEPARTMENTS] ORDER BY [DEP_ID] DESC"></asp:SqlDataSource>
        <asp:SqlDataSource ID="Departments_SqlDS" runat="server" ConnectionString="<%$ ConnectionStrings:ESchoolConnectionString %>"
            UpdateCommand="Update GRADES Set GRD_NAME=@GRD_NAME,GRD_DEPID=@DEP_ID WHERE [GRD_ID]=@GRDID"
            SelectCommand="SELECT GRADES.GRD_ID, GRADES.GRD_NAME, DEPARTMENTS.DEP_ID,DEPARTMENTS.DEP_NAME, GRADES.GRD_DEPID FROM DEPARTMENTS INNER JOIN GRADES ON DEPARTMENTS.DEP_ID = GRADES.GRD_DEPID ORDER BY GRADES.GRD_ID DESC" 
            DeleteCommand="Delete From Grades Where GRD_ID=@GRDID" >
            <UpdateParameters>
                <asp:Parameter Type="String" Name="GRD_NAME" />
                <asp:Parameter Type="Int32" Name="DEP_ID" />
                <asp:Parameter Type="Int32" Name="GRDID" />
            </UpdateParameters>
        </asp:SqlDataSource>
        <asp:GridView ID="GridView1" runat="server" AllowPaging="True" AllowSorting="True"
            AutoGenerateColumns="False" BackColor="White" BorderColor="#999999" BorderStyle="None"
            BorderWidth="1px" CellPadding="3" DataSourceID="Departments_SqlDS" Width="100%" DataKeyNames="GRD_ID" ForeColor="White" >
            <FooterStyle BackColor="#CCCCCC" Font-Size="12px" ForeColor="Black" />
            <RowStyle BackColor="#EEEEEE" Font-Size="12px" ForeColor="Black" />
            <Columns>
                <asp:BoundField DataField="GRD_ID" HeaderText="序号" InsertVisible="False" ReadOnly="True"
                    SortExpression="GRD_ID" />
                <asp:BoundField DataField="GRD_NAME" HeaderText="年级名" SortExpression="GRD_NAME" />
                <asp:TemplateField HeaderText="部名" SortExpression="GRD_DEPID">
                    <ItemTemplate>
                        <asp:Label ID="Label1" runat="server" Text='<%# Bind("DEP_NAME") %>' Width="121px"></asp:Label>
                    </ItemTemplate>
                    <EditItemTemplate>
                        <asp:DropDownList ID="DEP_DropDownList" DataSourceID="SqlDataSource1" DataTextField="DEP_NAME" DataValueField="DEP_ID" SelectedValue='<%# Bind("DEP_ID") %>' runat="server"></asp:DropDownList>
                    </EditItemTemplate>                
                </asp:TemplateField>
                <asp:CommandField ShowDeleteButton="True" ShowEditButton="True" CancelText="取消" DeleteText="删除" EditText="编辑" UpdateText="更新" />
            </Columns>
            <PagerStyle BackColor="Silver" Font-Size="12px" ForeColor="Black" HorizontalAlign="Right" />
            <SelectedRowStyle BackColor="#008A8C" Font-Bold="True" ForeColor="White" />
            <HeaderStyle BackColor="Silver" Font-Bold="True" Font-Size="12px" ForeColor="White" />
            <EditRowStyle Font-Size="12px" ForeColor="#FF8000" />
            <AlternatingRowStyle BackColor="Gainsboro" />
        </asp:GridView>
    </form>
</body>
</html>