If you get this error while running Visual Studio Xaml Editor then do the following - Go to Installation folder For me it is C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE - then type devenv.exe /setup Then restart the Visual Studio and it should fix the error. Although there are many blogs which talk about uninstalling and reinstalling .NET Framework 3.5 sp1 or few Windows Updates ends with 09 or 10 but this will fix it uninstalling and installing do not help.