public async Task DeleteCourseByCourseIdAsync(long courseId)
{
if (courseId < 0)
{
throw new ArgumentException();
}
//删除course下的class
await _iClassService.DeleteClassByCourseIdAsync(courseId);
//删除course下的seminar
await _iSeminarService.DeleteSeminarByCourseIdAsync(courseId);
//删除course
await _iCourseDao.DeleteCourseByCourseIdAsync(courseId);
}