get_postBackElement() 总是未定义

Question

<asp:ScriptManager ID="ScriptManager1" ScriptMode="Release" runat="server">
</asp:ScriptManager>
<script type="text/javascript">
    Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);

    function BeginRequestHandler(sender, args) {
        var elem = args.get_postBackElement();
        alert("begin " + elem.value);
    }
    function EndRequestHandler(sender, args) {
        alert("end request handler");
    }
</script>

这是我检索从我的 UpdatePanel 触发的回发元素的简单尝试。我的更新面板如下所示：

            <asp:UpdatePanel ID="UpdatePanel_Project" UpdateMode="Conditional" runat="server">
                <ContentTemplate>
                    <asp:DropDownList ID="DropDownList_Project" runat="server">
                    </asp:DropDownList>
                </ContentTemplate>
                <Triggers>
                    <asp:AsyncPostBackTrigger ControlID="DropDownList_Customer" />
                </Triggers>
            </asp:UpdatePanel>

我相信我做的一切都是正确的......

有任何想法吗？

Answer 1

您的回发触发器是什么？这个控件好像缺失了DropDownList_Customer

 <asp:AsyncPostBackTrigger ControlID="DropDownList_Customer" />