public void Add_NonEmptyMessageOfKnownUser_ShouldSaveMessage() { var user = fixture.Create <User>(); userReader.Stub(r => r.User).Return(user); var messageText = fixture.Create <string>(); sut.Add(messageText); messageRepository.AssertWasCalled(r => r.Save(Arg <Message> .Matches(m => m.Text == messageText && m.User == user))); }