I started developing a hobby project at home to try using TDD. I pretty quickly came to an issue. The app needs to store and update information in the ApplicationData directory. I want to unit test this functionality, but I don't want it mucking with my files on disk from when I last used the app.
When I first hit this I resolved it by rewriting some code that shouldn't have been hitting the disk anyway and using an in-memory buffer to test that serialization works. However at some point I did actually want to test that the information gets correctly persisted to disk.
The solution I came up with for this case is to use RunAs to run NUnit-gui as a different user. I created a Tester Account on my machine with limited access and added to the TestFixture for the class that writes to disk a [TestFixtureSetUp] method that throws if the user is not my special tester account. If I run NUnit in a different account the tests that touch files simple won't be run.