As you can read in CA1001: Types that own disposable fields should be disposable. Pretty solid advice, but for some reason the mstest runner does not dispose of classes that implement IDisposable. A possible workaround is to apply a TestCleanupAttribute to the Dispose method, but this is really contradictory with the “Shared test fixture” approach mstest uses. Imho, there is only one clean solution: use a decent testing framework instead.