public static RegistrationResult Register(int cartId) { RegistrationResult result = new RegistrationResult(); ShoppingCart cart = ShoppingCartRepository.GetInstance().Get(cartId); Student student = cart.Student; foreach (Course course in cart.Courses) { if (course.EnrolledStudents.Contains(student)) { result.Success = false; result.Errors.Add($"Student {cart.Student.Id} already enrolled in course {course.Name}"); } } if (result.Success) { foreach (Course course in cart.Courses) { course.EnrolledStudents.Add(cart.Student); } } return(result); }