Problem: You want to run NUnit tests in a class library (dll). These test rely on an application configuration file (app.config) for some settings, i.e. custom appSettings or database connection strings. The code compiles and runs fine by itself, but your unit tests always fail. Attaching the Visual Studio debugger to NUnit and stepping through the code, you see that the config seems to be ignored.

Solution: This is a perfectly reasonable thing to do, and it works fine. That is, it works if your "project" in NUnit consists of the assembly, not the Visual Studio Project. If you setup the NUnit project by clicking on Project > Add VS Project, then for some reason the configuration file will be ignored. If, however, you simply drag your assembly file into NUnit, then it should work as expected.