public IActionResult AddCourse(int studentId, int courseId)
        {
            string errorMessage = string.Empty;

            try
            {
                service.AssignCourseToStudent(courseId, studentId);

                return(RedirectToAction("Index"));
            }
            catch (Exception ex)
            {
                errorMessage = ex.Message;
            }

            TempData["ErrorMessage"] = errorMessage;

            return(RedirectToAction("AvailableCourses"));
        }