问将网格视图导出到excel文件后，我的下拉列表将不起作用？

EN

   protected void Button1_Click(object sender, EventArgs e)
   {

      // HttpContext.Current.Response.Clear();
       HttpContext.Current.Response.AddHeader(
           "content-disposition", string.Format("attachment; filename={0}", "Kontratat-" + DateTime.Now.ToShortDateString() + ".xls"));
       HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";


       StringWriter sw = new StringWriter();
       HtmlTextWriter hw = new HtmlTextWriter(sw);


       HtmlForm frm = new HtmlForm();
       hw.WriteLine("<center><b><u><font size='5'>Kontratat</font></u></b></center>");//will be displayed in excel as a heading.
       GridView1.Parent.Controls.Add(frm);
       frm.Controls.Add(GridView1);
       frm.RenderControl(hw);
       HttpContext.Current.Response.Write(sw.ToString());
       HttpContext.Current.Response.Flush();
       HttpContext.Current.Response.End();

   }            

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<Triggers>
    <asp:AsyncPostBackTrigger ControlID="DropDownList1" EventName="SelectedIndexChanged" />
   <asp:PostBackTrigger ControlID="Button1" />
</Triggers>
<ContentTemplate>
    <asp:GridView ID="GridView1" runat="server" BackColor="#CCCCCC" BorderColor="#999999" BorderStyle="Solid" BorderWidth="1px" CellPadding="4" CellSpacing="2" ForeColor="Black">
    </asp:GridView>
</ContentTemplate>