Indeed, you are perfectly right, please excuse that - I hadn't check the product line.
Back to the problem, I just did some more digging about this error as I could not reproduce it on a test machine. I found we had a similar report before
and the solution a customer reported was that their project was referring older versions of the RadControls assemblies than the ones they had installed.
Can you check the property boxes of the RadControls references of your project? Do they have the version 2011.2.718.1040?
Another thing that could give us some more information is:
1) Create a new Silverlight project
2) Drag a RadControl to the page
3) Check the version of the assemblies that got added to the project
If it different than 2011.2.718.1040, that would mean that the problem happens because the versions of the ToolboxItem assemblies differs from the version of the assemblies in your other project. You could run the ToolboxConfigurator (Telerik -> RadControls for Silverlight -> Launch Toolbox Configurator
menu item in Visual Studio) to update the Toolbox to the latest version.
Let me know if these helped.
the Telerik team
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