public async Task <Course> GetCourseByCourseIdAsync(long courseId)
{
if (courseId < 0)
{
throw new ArgumentException();
}
var course = await _iCourseDao.GetCourseByCourseIdAsync(courseId);
//没查到该门课
if (course == null)
{
throw new CourseNotFoundException();
}
return(course);
}