/// <summary>
/// 增订教材；
/// </summary>
/// <param name="sender">事件发送方</param>
/// <param name="e">选课事件参数</param>
public void AddBookOrder(object sender, SelectCourseEventArgs e) //注册至事件的函数的签名必须与委托定义一致；
{
this.BookOrderCount++;
Student student = sender as Student;
WriteLine($"\n已为学生{student.Name}增订教材，当前共需订购教材{this.BookOrderCount}本。");
}
public event SelectCourseEventHandler SelectCourseEvent; //定义事件；事件即多播委托；事件必须定义在类内部；
/// <summary>
/// 触发选课事件；
/// </summary>
/// <param name="courseName">课程名称</param>
private void OnSelectCourse(string courseName)
{
var e = new SelectCourseEventArgs(courseName);
this.SelectCourseEvent?.Invoke(this, e); //触发事件，即调用多播委托中注册的所有事件处理函数；事件必须在类内部触发；还需根据需要创建事件参数；
}
/// <summary>
/// 通知；
/// </summary>
/// <param name="sender">事件发送方</param>
/// <param name="e">选课事件参数</param>
public static void InformAfterSelectCourse(object sender, SelectCourseEventArgs e) //注册至事件的函数的签名必须与委托定义一致；
{
Student student = sender as Student;
WriteLine($"\n发送短信至手机{student.PhoneNumber}：已成功选修《{e.CourseName}》。\n");
}
