<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?

<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?

• javascript
• asp

• 1 To help verify, could you debug and see from the server what Request.Form["__EVENTTARGET"] is? – Brian Mains Commented Aug 17, 2011 at 12:08
• I am having this exact same problem. Did you ever find a resolution? – personaelit Commented Apr 26, 2012 at 1:43
• @Jim - I didn't find an ASP.NET solution to this, as I hoped. I went with jQuery all the way (for that functionality). – KristianB Commented Apr 26, 2012 at 7:01
• @Jim The problem still exists in Microsoft's code. Please view my below answer. – Ashraf Sabry Commented Dec 22, 2013 at 11:12
• @AshrafSabry Since Microsoft isn't going to fix their bug, my answer is the only way to use ClientIDMode="Static" without postback issues. – Brad Bamford Commented Feb 7, 2015 at 23:01

3 Answers 3

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.

What is your postback trigger ? This control seems to be missing DropDownList_Customer

 <asp:AsyncPostBackTrigger ControlID="DropDownList_Customer" />

I finally solved this pain, here's my solution.

Basically we just need to override Microsoft's _uniqueIDToClientID function so it doesn't ignore our Static client Ids on postback elements.

You'll just need to add the following code at the bottom of your page.

if (Sys.WebForms.PageRequestManager) {
  var prm = Sys.WebForms.PageRequestManager.getInstance();

  prm._uniqueIDToClientID = function (uniqueID) {
    var clientID = uniqueID.replace(/\$/g, "_");
    if (document.getElementById(clientID)) {
      return clientID;
    }

    var lastDollar = uniqueID.lastIndexOf("$");
    if (lastDollar === -1) {
      return clientID;
    }

    if (lastDollar+1 > uniqueID.length) {
      return clientID;
    }

    var staticID = uniqueID.slice(lastDollar + 1);

    return document.getElementById(staticID) ? staticID : clientID;
  };
}

Now, get_postBackElement() in your BeginRequestHandler will no longer be undefined!

Just make sure our code is executed after Microsoft's MicrosoftAjaxWebForms.js because we are overriding its _uniqueIDToClientID function.

Note: My function always returns the default WebForms ClientID if the element exists on the page. Only if the element cannot be found on the page, does it check to see if an element with a static ID exists on the page. If it exists, the staticID is used, otherwise it defaults back to the default WebForms ClientID.