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🐞 Fixing AjaxControlTookit Unexpected Page Reload in ASP.NET Web Form Application

·906 words·5 mins
Hong Konger.

😨 Problem #

TL;DR: see Summary and the working code snippet if you want to try it right away.

In my company there is an old Web Form Website running .NET Framework 3.5. I migrated it to an ASP.NET Web Form Application running .NET Framework 4.7.2 few months ago. The website is running just like the old one but recently a user reported that:

a <select> list which should triggers an AJAX call and refresh part of the page does not work. Instead, the page is reloaded everytime when the item in the <select> list is changed.

I used almost a day to figure out what’s (probably) happened and I hope that this article will be helpful to someone in the future who (still) encounter this problem.

Expected vs Actual Behavior #

There is a <form> containing a <select> list (generated by <asp:DropDownList>). When the selected option is changed, an AJAX call will be fired and new items will be fetched. Besides, there is a text area for user to input remarks. A submit button will submit the form.


Expected #

The page will not be reloaded when the selected option is changed. The content in the text area retains.

Actual #

Once the selected option is changed, a form POST is fired (instead of an AJAX call). The content in the text area is therefore being cleaned.

This is related to AJAX calls so the first thing I checked was how the AJAX call was initiated.

😀 Solution (Part 1): Updating AjaxControlToolkit #

In the .aspx file I saw a <asp:UpdatePanel> and a <asp:DropDownList>. This is the simplified structure of that part:

<asp:UpdatePanel runat="server" ID="UpdatePanel2" UpdateMode="Conditional" ChildrenAsTriggers="true">
    <asp:DropDownList runat="server" ID="ddItemTypes" AutoPostBack="true" OnSelectedIndexChanged="ddItemTypes_OnSelectedIndexChanged" />

AutoPostBack is there so the app should use AjaxControlToolkit to make AJAX calls (I guess that’s the standard right?).

Nuget Package Update #

I checked the package.config:

<package id="AjaxControlToolkit" version="4.1.50508" targetFramework="net35" />

Looks like the targetFramework is not right. Maybe it’s time to update the package too.

<package id="AjaxControlToolkit" version="20.1.0" targetFramework="net472" />

Some Breaking Changes #

AjaxControlToolkit has some breaking changes since version 15+. In short, you need to:

  1. Uninstall the old version and install the new version of AjaxControlToolkit.

  2. Change <asp:ToolkitScriptManager> to <asp:ScriptManager>

  3. Remove some unused configs in web.config (see this)

  4. [Optional] Install AjaxControlToolkit.HtmlEditor.Sanitizer

    • Change the namespace AjaxControlToolkit.HTMLEditor to AjaxControlToolkit.HtmlEditor

    • Change the namespace AjaxControlToolkit.HTMLEditor.ToolbarButton to AjaxControlToolkit.HtmlEditor.ToolbarButtons

You may also read the official complete migration guide.

After I upgrade the Nuget package, the AJAX calls was working partially: It only works on Chromium-base browsers like Chrome and Edge. However, it does not work on Firefox (85.0) and Safari (14.0.1).

😐 😐 😐

Different __doPostBack() in Firefox? #

This is what I got from Firefox’s dev tool:

call stack form Firefox image-20210212140706748

The initiator is from __doPostBack() in the .aspx page. In Chrome, it’s from ScriptResource.axd. Below is the expected call stack: the call is an XHR request fired from ScriptResource.axd.

Expected call stack image-20210212141300293

Looks like the stack items from bottom to _doPostBack() are the same. I therefore dug into _doPostBack() and eventually found this piece of code in ScriptResource.axd:

if (!this._postBackSettings.async) {            
    form.onsubmit = this._onsubmit;            
    this._originalDoPostBack(eventTarget, eventArgument);            
    form.onsubmit = null;            

if this._postBackSettings.async is false, _originaldoPostBack() will be called and that is the __doPostBack() function defined in the .aspx page. This will trigger a page reload instead of an AJAX call.

Furthermore, I found that the asyncTarget in _postBackSettings is null but in Chrome that target is the <select> list.

In Firefox (v85.0) the javascript event initiated is somehow keep propagating to a higher level than the <select> list (probably to the form’s level) and therefore being treated as a form POST instead of an AJAX call.

😀 Solution (Part 2): Adding <asp:AsyncPostBackTrigger> #

I did some research related to the Firefox-specific page reload and seems none of them were talking about the issue I met. I tried to study the fundamentals of this PostBack behaviour.

Finally I got this article Avoid (Prevent) Page refresh (PostBack) after SelectedIndexChanged is fired in ASP.Net DropDownList. It suggested what I might missing is an asp:AsyncPostBackTrigger.

Also, this StackOverflow answer menitoned the asp:AsyncPostBackTrigger thing.

So my understanding is:

When using the ScriptManager (i.e. the one we introduced when updating the AjaxControlToolkit), we need to define an asp:AsyncPostBackTrigger Trigger in order to make the call AJAX.

Therefore, I added a Trigger section to the .aspx file:

  <asp:AsyncPostBackTrigger ControlID="ddItemTypes" EventName="SelectedIndexChanged" />

This Trigger section should be placed under the same <asp:UpdatePanel> with the <asp:DropDownList>. Here is the complete snippet:

The Working Code Snippet #

<asp:UpdatePanel runat="server" ID="UpdatePanel2" UpdateMode="Conditional" ChildrenAsTriggers="true">
    <asp:DropDownList runat="server" ID="ddItemTypes" AutoPostBack="true" OnSelectedIndexChanged="ddItemTypes_OnSelectedIndexChanged" />
    <asp:AsyncPostBackTrigger ControlID="ddItemTypes" EventName="SelectedIndexChanged" />


  1. EventName is optional here but since I just want this trigger to listen the SelectedIndexChanged event, I add them all.

  2. The ControlID has to be an exact match with the ID of <asp:DropDownList>.

After I added the <asp:AsyncPostBackTrigger>, all the browsers including Chrome, Edge, Firefox (85.0) and Safari (14.0.1) are working.

Summary #

When your .NET 4.5+ ASP.NET Web Form Application using AjaxControlToolkit does not work as expected and cause the controls like <asp:DropDownList> cannot fire an AJAX call (but triggers a form POST), make sure that:

  1. The AjaxControlToolkit Nuget package is up to date (and did the proper migration steps here).

  2. The <asp:UpdatePanel> section should contain a <Triggers> section. In that section there is an <asp:AsyncPostBackTrigger> with ControlID same as the ID of your control (which is also placed under the same UpdatePanel section).


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