The Visual Studio designer classes of the Telerik® UI for ASP.NET Ajax are located in the Telerik.Web.Design assembly.
During the installation of Telerik® UI for ASP.NET Ajax, the Telerik.Web.Design assembly gets added to the GAC. Starting a page with any Telerik control on it in design mode, the Visual Studio IDE searches the designer class of the Telerik control and the Telerik.Web.Design assembly respectively. Normally, when the assembly is located in the GAC, the Visual Studio IDE loads it properly and displays the control designers.
There are several cases that errors occur in design time.
Here is a list of the circumstances when this is likely to happen:
Manual upgrade - It is often faster to download the new Service Pack or Latest Internal Build as a zip file and replace the Telerik.Web.UI reference manually. Forgetting to add the Telerik.Web.Design assembly to the GAC breaks the Visual Studio design time, because the Visual Studio IDE cannot load the appropriate design assembly version. The error message can be fixed by either adding the proper Telerik.Web.Design assembly to the GAC, or adding it to the local BIN folder - the Visual Studio IDE will resolve the designer classes properly. A Visual Studio IDE restart is recommended to avoid loading duplicate shadow copied assemblies (Visual Studio IDE default behavior).
Using Visual Studio 2008 without Service Pack 1 applied - The pre-SP1 Visual Studio 2008 IDE has a bug with the proper assembly loading. The Telerik.Web.Design assembly exists in the GAC and the IDE loads it, but it cannot resolve the dependence between the Telerik.Web.Design and Telerik.Web.UI assemblies properly. To fix the design time error in this case, you could either apply the Visual Studio SP1 upgrade, or copy the Telerik.Web.UI assembly to the [VisualStudioInstallationFolder]\Common7\IDE\PublicAssemblies folder. This way the Visual Studio IDE would be able to properly resolve the reference between the two assemblies and load the designers.
Using the second approach (copying the Telerik.Web.UI assembly to the [VisualStudioInstallationFolder]\Common7\IDE\PublicAssemblies folder) could itself cause the problem described above. If you open a project, using a different version of the Telerik.Web.UI assembly than the one in the PublicAssemblies folder, the IDE could display an InvalidCast error message. It is suggested that you ensure the version of the Telerik.Web.UI assembly used by your project and its version in the PublicAssemblies folder are the same before loading the IDE.
This instance of the error usually happens when upgrading Telerik® UI for ASP.NET Ajax.
To help avoid these problems, the Project Wizard detects the existence of the Telerik.Web.Design assembly in the GAC. If the assembly is not found in the GAC, the wizard will suggest to add it to the Global Assembly Cache:
In case Telerik.Web.UI.Design.dll is not found in the selected distribution the wizard will display a warning of possible design time problems:
Adding the design assembly to the GAC requires elevated privileges . In case you have User Account Control enabled on your machine you will have to grant permissions to the GAC Utility