public async Task AddProductToCustomerrWishlistAsync_ShouldReturnExistingProduct_WhenProductIsAlreadyInWishlist() { //Arrange string productId = "1bf0f365-fbdd-4e21-9786-da459d78dd1f"; _customerService.GetCustomerAsync(customerId).Returns(existingCustomer); var existingList = new List <WishListProduct>() { new WishListProduct() { ProductId = productId } }; _wishlistRepository.GetCustomerWishlistAsync(customerId).Returns(existingList); //Act var sut = new WishlistService(_productRest, _customerService, _wishlistRepository); var product = await sut.AddProductToCustomerrWishlistAsync(customerId, productId); //Assert Assert.NotNull(product); await _wishlistRepository.DidNotReceive().InsertWishlistProductAsync(Arg.Any <int>(), Arg.Any <WishListProduct>()); Assert.Equal(productId, product.ProductId); }