This article shows how to resolve common design-time errors that appear in the Visual Studio designer.
Such issues might occur in the following situations.
After an upgrade to a newer version of UI for Silverlight. Or after a downgrade to a previous version.
Referencing Telerik dlls from different locations on the file system. For example, if you get the Telerik.Windows.Controls.dll assembly from C:\TelerikDlls and Telerik.Windows.Navigation.dll from C:\TelerikAssemblies.
To resolve this you can use the following steps.
Make sure that all Telerik dlls are the Xaml or the NoXaml version. In other words avoid mixing them. Choose only one type.
Make sure that all Telerik dlls come from the same place (directory).
Make sure that only the StyleManager or the Implicit Styles theming mechanism is used. If you reference XAML dlls use StyleManager. If you reference NoXaml, use Implicit Styles.
Close all .xaml tabs in Visual Studio.
Close Visual Studio.
Clear Visual Studio designer's cache. To do this, go to the following directory: %LOCALAPPDATA%\Microsoft\VisualStudio\14.0\Designer\ShadowCache. Then delete all subfolders.
The folder named 14.0 in the shadow cache directory path corresponds to the Visual Studio version that you are using. The name of this folder could be different on your machine.
Open your project's local folder and manually delete the bin and obj folders.
Open the project in Visual Studio again.
Clean and Rebuild the project.