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Known Issues

This article provides links to specific Knowledge Base articles covering scenarios for handling some of the most common Telerik UI for ASP.NET AJAX issues.

For faster navigation, the links are grouped into the following sets:

For more information on using Telerik UI for ASP.NET AJAX and Microsoft AJAX with the strict Content Security Policy mode, refer to the Content Security Policy Mode article.



Web Resources

If you are using an ASP.NET server control with rich client-side behavior, it is likely built to utilize web resources. Sometimes your page loads and the rich server-side control does not work at all, for example, the tree view does not expand, the grid cannot sort, and so on. Most of the times such issues occur because the JavaScript files of those controls have failed to load.

If your browser is configured to prompt on JavaScript errors, you may see an error message similar to the RadTreeView is undefined one.

If you are using Telerik UI for ASP.NET AJAX, receiving this message indicates that there might be a Web Resource related issue.

To identify the error that occurs, use either of the following approaches:

  • Manually request the WebResource handler.

    View the rendered output of your page and get the URL of the offending script tag, for example:

      <script type="text/javascript"

    If you paste that URL in the address bar of your browser, specifically after the domain and folder, the web server will serve back the content of that web resource. If the issue is related to the HTTP handler of the web resource, you will see an error page saying that the server returned an HTTP error code 404 (not found) or 500 (server error).

  • Using web development tools to request the WebResource handler.

    Use an HTTP traffic sniffer tool like Google DevTools or Firefox FireBug to find out if the request to a web resource file has failed and what the exact error is.

The following list links to the most common issues related to web resources when working with Telerik UI for ASP.NET AJAX:


When working with Telerik UI for ASP.NET AJAX, note that as of R1 2018, the Telerik.Web.Design.dll assembly is no longer added by the automated .msi installer to the GAC. The Telerik.Web.Design.dll file is the assembly that contains the design-time code and if it is not referenced properly in your project, you may get design-time errors.

Previously, Telerik.Web.Design.dll was installed in the GAC by the automated installer of the Telerik UI for ASP.NET AJAX suite. Adding it to the GAC was necessary because of Visual Studio 2008 that could not otherwise read design-time code and has caused various issues with later versions because it prevented proper upgrades and broke references.

Some of the design-time issues linked below, which refer to the previous installation approach of the assembly, may require you to modify the GAC to remove duplicate assemblies. For more information, refer to the article on using the global assembly cache.

Visual Studio ToolBox


jQuery Integration

Unobtrusive validation uses the HTML5 attributes and jQuery for validation purposes. It is enabled by default for all .NET 4.5 projects and requires a jQuery library that is registered with the ScriptManager as jquery. To add this registration, install the AspNet.ScriptManager.jQuery NuGet package.

In addition, when the page contains a ScriptManager control, you need to instruct it to register jQuery before the WebForms.js and WebUIValidation.js core script files that provide the unobtrusive validation. For more information about this requirement, refer to the Unobtrusive validation breaks with a Script Manager on the page Microsoft Connect feedback item.

The unobtrusive validation is automatically configured when you create a web site through the .NET 4.5 Web Forms Site template of Visual Studio. If you use it with a RadScriptManager or in a Telerik website or web application template, however, a few common scenarios that require additional modifications may occur. For more information, refer to the troubleshooting scenarios on integrating jQuery.

Most of the issues related to the jQuery integration are related to the Unobtrusive Validation mode. You can avoid jQuery and validation problems with a single line in the web.config file that will disable the unobtrusive validation:

        <add key="ValidationSettings:UnobtrusiveValidationMode" value="None"/>


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