asp.net中 服务器控件中onselectedindexchanged 没有反应的解决方案

<form id="T_Add" runat="server"  >
 <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
                </asp:ScriptManager>
                <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                  <Triggers>  
        <asp:PostBackTrigger ControlID="LinkButton2" />  //这里是最重要的解决方法
         <asp:AsyncPostBackTrigger ControlID="DropStage" EventName="SelectedIndexChanged" />
</Triggers>
                <ContentTemplate>

<asp:LinkButton ID="LinkButton2" runat="server" onclick="LinkButton2_Click">LinkButton</asp:LinkButton>

<asp:DropDownList ID="DropStage" runat="server" CssClass="select01" 
                                    onselectedindexchanged="DropStage_SelectedIndexChanged" AutoPostBack="true">
                               
                               <asp:ListItem Value="">-选择-</asp:ListItem>
                                </asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</form>