Error: Could not load file or assembly Telerik.Web.UI or one of its dependencies

Problem

It is possible to receive the following error message after upgrading your web project with RadControls for ASP.NET AJAX to another version:
Could not load file or assembly 'Telerik.Web.UI, Version=XXXXX.X.XXXX.X, Culture=neutral, PublicKeyToken=XXXXXXXXXXXXX' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Description

The problem is due to the fact that the RadEditor's DialogHandler and SpellCheckHandler *httpHandlers *are registered in the web.config file with their fully qualified names and their versions are not updated.

Solution

After upgrading your project to another version, you should update not only the Telerik.Web.UI.dll in the /bin folder, but also the DialogHandler and SpellCheckHandler *httpHandlers *declarations in web.config file as shown below:

<httpHandlers>
      ...
      <add path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI, Version=2021.2.616.45, Culture=neutral, PublicKeyToken=121fae78165ba3d4"
        validate="false" />
      <add path="Telerik.Web.UI.SpellCheckHandler.axd" verb="*" type="Telerik.Web.UI.SpellCheckHandler, Telerik.Web.UI, Version=2021.2.616.45, Culture=neutral, PublicKeyToken=121fae78165ba3d4"
        validate="false" />
</httpHandlers>

You should write the fully qualified names only if the Telerik.Web.UI.dll is registered in the GAC. If the Telerik.Web.UI.dll file is placed in the /bin folder of your project you can register the SpellCheckHandler httpHandlers in the web.config file with their short names, e.g.:

<httpHandlers>
    ...
    <add path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI"
 validate="false" />
    <add path="Telerik.Web.UI.SpellCheckHandler.axd" verb="*" type="Telerik.Web.UI.SpellCheckHandler, Telerik.Web.UI"  validate="false" />
</httpHandlers>

Note: If the Telerik ASP.NET AJAX VSExtensions are installed then the DialogHandler and SpellCheckHandler handlers will be automatically registered in the web.config file when the Telerik > RadControls for ASP.NET AJAX > Upgrade Wizard menu is used.