You are posting in the Windows Phone 7 forums, but I'm assuming this is a Windows Phone 8 project you are trying to use the controls with.
If yes, then this error is usually due to the fact the projects reference have not been updated to the new version you just installed on your machine. You can repair this in one of two ways:
Upgrade Wizard (Recommended)
Use the project Upgrade Wizard which is part of the Telerik VSExtensions for Windows Phone. To do this open your project in Visual Studio, right click on the project, expand "Telerik UI for Windows Phone
" and select Upgrade Wizard
From there the project's old references will be replaced with the new installation's references. Go here in the
documentation for a step-by-step tutorial on how to use the wizard.
You can replace the DLLs manually by removing all the Telerik DLLs from your project's references. Now re-add the new DLLs by going to C:\Program Files (x86)\Telerik\UI for Windows Phone 8 Q2 2014\Binaries\WindowsPhone
After updating your project's references, the xmlns declaration to Telerik.Windows.Controls
should resolve and your project will build.
If it is still not working, please reply with the code you are using and I will be able to investigate further.
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