get_postBackElement() is always undefined

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>

This is my simple attempt to retrieve the postback element, triggered from my UpdatePanel. My update panel looks like this:

            <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>

I believe I have did everything correctly...

Any ideas?

Answer 1

You have to set the ClientIDMode property value of the control (the DropDownList_Customer drop down list in this case) to AutoID. Please view my reply here.