public void EnrolInstructorToCourse(int courseID, int instructorID)
{
// check if course exists
ICourseRepository courseRepo = new CourseRepository();
if (!courseRepo.IsCourseExist(courseID))
{
// course not exists
throw new CourseNotFondException(courseID + " is not a registered course.");
}
courseRepo.EnrolInstructorToCourse(courseID, instructorID);
}