public void Init() { // Arrange var mockRepo = new Mock <IStoreRepository> (); var mockCartRepo = new Mock <ICartRepository> (); category = new Category("Cat1"); mockRepo.Setup(s => s.AddCategory(category)); mockRepo.SetupGet(r => r.Categories).Returns(new List <Category> { category }); store = new Store(mockRepo.Object, mockCartRepo.Object); // Act store.AddCategory(category); }