public async Task <long> InsertCourseByUserIdAsync(long userId, Course course)
{
if (userId < 0)
{
throw new ArgumentException();
}
//根据userId找出teacher
var teacher = await _iUserService.GetUserByUserIdAsync(userId); //会抛出ArgumentException和UserNotFoundException
course.Teacher = teacher;
var courseId = await _iCourseDao.InsertCourseByUserIdAsync(course);
return(courseId);
}