I’m having issues getting UpdatePanel working in a large, existing solution. I have a

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Asked: May 22, 20262026-05-22T22:18:02+00:00 2026-05-22T22:18:02+00:00

I’m having issues getting UpdatePanel working in a large, existing solution. I have a

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I’m having issues getting UpdatePanel working in a large, existing solution. I have a sample page (below), which works in a freshly created demo website, but not when added to the existing target website.

The functionality is to isolate a dropdown’s auto-postback, so I don’t lose the contents of a FileUpload ASP control (it does this for security reasons, with some solutions described here)

In the target (non-demo) site, the control adds to the page fine (inc intellisense), and the page renders – but changing the drop down still performs a postback, rather than ajax-ifying the dropdown box.

The target solution mentioned was previously upgraded from ASP.NET v1.1, so I’m wondering if there’s something I’m missing in the configuration?

The only difference I can find in the rendered HTML source is that the non-working version doesn’t add the PageRequestManager, e.g.:

<script type="text/javascript"> 
//<![CDATA[
Sys.WebForms.PageRequestManager._initialize('ctl02', document.getElementById('form1'));
Sys.WebForms.PageRequestManager.getInstance()._updateControls(['tctl03'], [], [], 90);
//]]>
</script>

Sample page:

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">

    <asp:ScriptManager ID="ScriptManager2" runat="server" />
    <div>
        <asp:UpdatePanel ID="UpdatePanel2" runat="server">
            <ContentTemplate>
                <asp:DropDownList runat="server" ID="TestDropDown" AutoPostBack="true" OnSelectedIndexChanged="TestDropDown_SelectedIndexChanged">
                    <asp:ListItem>One</asp:ListItem>
                    <asp:ListItem>Two</asp:ListItem>
                    <asp:ListItem>Three</asp:ListItem>
                </asp:DropDownList>

                <asp:Literal runat="server" Text="Original state" ID="litText" />
            </ContentTemplate>
        </asp:UpdatePanel>

        <asp:FileUpload ID="FileUpload2" runat="server" />
    </div>

    </form>
</body>
</html>

And in the code behind:

protected void TestDropDown_SelectedIndexChanged(object sender, EventArgs e)
    {
        litText.Text = "Ajax update. The file details should still be present below";
    }

I can confirm that the ScriptManager tag adds the following to the page source, so I assume the Ajax Toolkit has been added:

if (typeof(Sys) === 'undefined') throw new Error('ASP.NET Ajax client-side framework failed to load.');

And that we have the following sections in web.config:

<compilation defaultLanguage="c#" debug="true">
      <assemblies>
        <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

[...]

<pages>
    <controls>
    <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, [...]
    <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions" [...]

[...]

<httpHandlers>
  <remove verb="*" path="*.asmx"/>
  <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
  <add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
  <add verb="GET,HEAD" path="ScriptResource.axd" validate="false" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
</httpHandlers>
<httpModules>
  <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
</httpModules>

The sample works in the demo site without the <Triggers> section, but I’ve tried adding it to the target site, to no avail.