public void CustomerLoanTest() { var customerManagerMock = new Mock <ICustomerManager>(); customerManagerMock.Setup(m => m.GetCustomerByNumber(It.IsAny <int>())) .Returns(new Customer { CustomerNr = 8, AmountOfBooksLoaned = 2 }); var customerAPI = new CustomerAPI(customerManagerMock.Object); var successfull = customerAPI.BookStatusLoan(8, 2, true); Assert.AreEqual(BookLoanStatus.OK, successfull); customerManagerMock.Verify(m => m.AddCustomer(It.IsAny <int>()), Times.Once); }