public void Adding_An_Account_To_An_Exisiting_User_Returns_One_Account_For_User() { var accountGuid = Guid.NewGuid(); var fakeReaderWriter = A.Fake <IReaderWriter>(); A.CallTo(() => fakeReaderWriter.ReadEnumerable <User>(_path)).Returns(_users); var userRepo = new UserRepo(fakeReaderWriter, _path); userRepo.AddAccountToUser(_user.UserGuid, accountGuid); A.CallTo(() => fakeReaderWriter.WriteEnumerable(A <string> .Ignored, A <IEnumerable <User> > .Ignored)).MustHaveHappened(Repeated.Exactly.Times(1)); Assert.That(1, Is.EqualTo(userRepo.GetUser(_user.UserGuid).AccountGuid.Count)); }