遭遇['GridView' must be placed inside a form tag with runat=server.]

　　悲剧的一个下午，在帮同事完善离职前留下的项目的时候，碰到这么一个稀奇古怪又不知道怎么解决的问题.

　　在使用GridView自带的直接导出Excel的功能的时候，发生了错误：　　

　　网上查阅了一些资料，但是都不是我的情况，代码很简单，也很明了...

　　前台：

 <form id="form1" runat="server">
    <div>
        <asp:GridView ID="gdvCustomer" runat="server" AllowPaging="True" 
            AutoGenerateColumns="False" DataSourceID="SqlDataSource1" >
            <Columns>
                <asp:BoundField DataField="ID" HeaderText="ID" SortExpression="ID" />
                <asp:BoundField DataField="NAME" HeaderText="NAME" SortExpression="NAME" />
                <asp:BoundField DataField="AGE" HeaderText="AGE" SortExpression="AGE" />
                <asp:BoundField DataField="CLASS" HeaderText="CLASS" SortExpression="CLASS" />
                <asp:BoundField DataField="GRADE" HeaderText="GRADE" SortExpression="GRADE" />
            </Columns>
        </asp:GridView>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:TestAccessDBConnectionString %>" 
            ProviderName="<%$ ConnectionStrings:TestAccessDBConnectionString.ProviderName %>" 
            SelectCommand="SELECT [ID], [NAME], [AGE], [CLASS], [GRADE] FROM [Customer]">
        </asp:SqlDataSource>

        <asp:Button ID="btnExport" runat="server" onclick="btnExport_Click" 
            Text="Export" />
        <br />
    </div>
    </form>

　　后台：　　

    /// <summary>
        /// 將Gridivew中的數據導出為Excel
        /// </summary>
        /// <param name="ctrl">Gridview名稱</param>
        /// <param name="FileName">要保存的文件名</param>
        public void ExportToExcel(Control ctrl, String FileName)
        {
            GridView gv = (GridView)ctrl;
            gv.AllowPaging = false;//禁用Gridview的分页
            gv.AllowSorting = false;//禁用Gridview排序
            gv.DataBind();
            HttpContext.Current.Response.Charset = "GB2312";
            HttpContext.Current.Response.Charset = "";
            HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.GetEncoding("GB2312");
            HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment;filename=" + FileName);
            HttpContext.Current.Response.ContentType = "application/ms-excel";
            HttpContext.Current.Response.Write("<meta http-equiv=Content-Type content=\"text/html; charset=GB2312\">");
            gv.Page.EnableViewState = false;
            StringWriter tw = new StringWriter();
            HtmlTextWriter hw = new HtmlTextWriter(tw);

            gv.RenderControl(hw);
            //gv.RenderBeginTag(hw);
            //gv.HeaderRow.RenderControl(hw);
            //foreach (GridViewRow row in gv.Rows)
            //{
            //    row.RenderControl(hw);
            //}
            //gv.FooterRow.RenderControl(hw);
            //gv.RenderEndTag(hw);

            HttpContext.Current.Response.Write(tw.ToString());
            HttpContext.Current.Response.End();

        }

        protected void btnExport_Click(object sender, EventArgs e)
        {
            ExportToExcel(this.gdvCustomer,"Customer.xls");
        }

　　使用gv.RenderControl(hw)导出Excel以前也都用过的啊，而且同样的方法，在这个页面的上一级跳转页面就有使用，而且导出Excel正常，纠结了我一个小时，实在找不出异常，最后走点弯路，把GridView一行一行的贴到Excel里.

　　前台不变，就是后台把GridView写入流的动作改变：

            gv.RenderBeginTag(hw);
            gv.HeaderRow.RenderControl(hw);
            foreach (GridViewRow row in gv.Rows)
            {
                row.RenderControl(hw);
            }
            gv.FooterRow.RenderControl(hw);
            gv.RenderEndTag(hw);

　　最后导出的Excel有点丑，表格的框线都不见了。。。只剩下光屁股的数据...唉！

　　哪位有解决办法或者曾经类似的解决经历么？求教！