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);
        }