What could be causing a System.TypeLoadException in a Visual Studio Unit Test?

I just banged my head against this one for an hour. The problem was that I had a command line project named Something.exe, which was using a class library project named Something.dll.

Happened to me too. In my case the problem arised because the tested project and the unit tests project had the same name. If this is your case too then rename one of the projects and rename output file name to fix it.