//更新学生名字和引发分数变化事件
        public void UpdateGrade(string nm, int grade)
        {
            GradeChangeEventArg arg = new GradeChangeEventArg(nm, grade);

            this.OnGradeChange(arg);
        }
        public void Student_Grade(object sender, GradeChangeEventArg arg)
        {
            Student st = sender as Student;

            Console.WriteLine($"{arg._name}班级{_classname}:同学{st._Name}收到期末成绩通知为{arg._Grade}");
        }
 protected void OnGradeChange(GradeChangeEventArg arg)
 {
     GradeChange?.Invoke(this, arg);
 }