public void ThrowWhenUserIsNull() { var mockedUnitOfWork = new Mock <IUnitOfWork>(); var mockedRepository = new Mock <IRepository <Discussion> >(); var mockedDiscussion = new Mock <Discussion>(); var discussionsService = new DiscussionsService(mockedRepository.Object, mockedUnitOfWork.Object); Assert.That(() => discussionsService.RemoveUserFromDiscussion(mockedDiscussion.Object, null), Throws.ArgumentNullException.With.Message.Contains("user")); }