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

            mock.Setup(a => a.Courses.Get(-1)).Returns((Course)null);

            //Act
            var service = new CourseService(mock.Object);

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