Unknown Server Tag or Could Not Load File or Assembly Errors Occur

Updated over 1 year ago

Environment

Product

Progress® Telerik® UI for ASP.NET AJAX

Description

When a project using Telerik controls is opened and a page is loaded, a "Unknown Server Tag" or "Could not load file or assembly 'Telerik.Web.UI, version='XXXXX'..." error occurs.

Error Message

Cause

Such errors usually occur when the project does not contain a reference to the Telerik.Web.UI assembly, or the reference is broken.

The possible causes for the errors are the following:

• There is a GAC-reference for the Telerik.Web.UI assembly in the project and the GAC has been updated, that is, an old Telerik.Web.UI assembly is deleted and a new one is added to the GAC. As a result, the Telerik.Web.UI assembly referenced by the website does not exist in the GAC.

• There is a Bin reference for the Telerik.Web.UI assembly in the project, which has been deleted for some reason. As a result, Visual Studio cannot find the Telerik.Web.UI assembly.

• The reference to the Telerik.Web.UI assembly, either in the Register directive or in the web.config file, or in the Telerik HTTP handlers or modules registrations in web.config, contains a hard-coded version number which does not match the version of the Telerik assembly referenced in your project.

Solution

To solve this issue, use the respective approach:

• Update the reference to the Telerik.Web.UI assembly in your project.

• Update the GAC reference:

  • (Manually) Edit the web.config and/or the ASPX/ASCX/Master/UserControl files to reflect the assembly version change. You can also skip to specify the exact assembly version to prevent future conflicts.

  • (Automatically) Use the Property pages. You will still need to update the <%@ Register Assembly="Telerik.Web.UI"… %> directives in your pages manually.

    

    

    Note that to avoid the need to manually modify the <%@ Register Assembly="Telerik.Web.UI"… %> directives, it is recommended that you register the tag prefix in the web.config instead, as shown in the screenshot below.

    

• Update the Bin reference:

  1. Browse to the Telerik.Web.UI assembly through the Property pages. Make sure to choose the assembly from the correct folder (e.g. bin35, bin40, bin45 or bin462) depening on the .NET version of your project.

     

     Note that when browsing to the Telerik.Web.UI assembly in the context of a website project, Visual Studio will always add a GAC reference if it finds the same version of the Telerik.Web.UI assembly.

  2. Use the Convert To Telerik project menu item available under the Telerik > Telerik® UI for ASP.NET AJAX menu in Visual Studio, which is available when the Visual Studio Extensions feature has been selected during the Telerik UI for ASP.NET AJAX installation.