If have some unitests in Visual Studio 2015 or 2013 but you cannot see them in your “Test Explorer window”
If you look in your output window, (under “Show output from:” > “test)
“An exception was thrown while initializing part “Microsoft.VisualStudio.TestWindow.UI.TestWindowToolWindowControl”.“
The solution is just to delete the VS2013/VS2015 cache folder, located either in
%LOCALAPPDATA%\Microsoft\VisualStudio\12.0\ComponentModelCache
For VS2013, or
%LOCALAPPDATA%\Microsoft\VisualStudio\14.0\ComponentModelCache
For VS2015