public void Give_error_when_second_time_added_same_product() { var mock = new Mock <IManagerDal>(); mock.Setup(t => t.IsExist(It.IsAny <Expression <Func <int, bool> > >())).Returns(true); MoqManager moqManager = new MoqManager(mock.Object); moqManager.Add(It.IsAny <int>()); }