public void DeleteOrderValidationTest(int?id)
        {
            //Arrange
            var mockUow = new Mock <IUnitOfWork>();

            mockUow.Setup(a => a.Orders.Get(-1)).Returns((Order)null);

            //Act
            var service = new RentService(mockUow.Object);

            //Assert
            Assert.Throws <ValidationException>(
                () =>
                service.DeleteOrder(id));
        }