详解C#之事件

/// <summary>
  /// 事件附加消息
  /// </summary>
  public class NewMailEventArgs:EventArgs{
    private readonly string m_from,m_to,m_subject;
    public NewMailEventArgs(string from,string to,string subject){
      m_from=from;
      m_to=to;
      m_subject=subject;
    }
    // 发件人
    public string From { get{return m_from;} }
    // 接收人
    public string To { get{return m_to;} }
    // 主题
    public string Subject{get{return m_subject;}}
  }

namespace System
{
  //
  // Summary:
  //   Represents the base class for classes that contain event data, and provides a
  //   value to use for events that do not include event data.
  public class EventArgs
  {
    //
    // Summary:
    //   Provides a value to use with events that do not have event data.
    public static readonly EventArgs Empty;

    //
    // Summary:
    //   Initializes a new instance of the System.EventArgs class.
    public EventArgs();
  }
}

 public class MailManager{
    // 定义事件成员
    public event EventHandler<NewMailEventArgs> NewMail;
    
  }

namespace System
{
  //
  // Summary:
  //   Represents the method that will handle an event when the event provides data.
  //
  // Parameters:
  //  sender:
  //   The source of the event.
  //
  //  e:
  //   An object that contains the event data.
  //
  // Type parameters:
  //  TEventArgs:
  //   The type of the event data generated by the event.
  public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);
}

public class MailManager{
    // 定义事件成员
    public event EventHandler<NewMailEventArgs> NewMail;
    // 定义负责引发事件的方法来通知已登记的对象
    protected virtual void OnNewMail(NewMailEventArgs e){
      // 将字段复制到一个临时变量，避免多线程情况中这个成员被移除
      EventHandler<NewMailEventArgs> temp=Volatile.Read(ref NewMail);
      if(temp!=null) temp(this,e);
    }

    // 接受附加信息并调用引发事件的方法来通知所有登记的对象
    public void SimulateNewMail(string from,string to,string subject){
      NewMailEventArgs e=new NewMailEventArgs(from,to,subject);
      OnNewMail(e);
    }
  }

public class Fax{
    public Fax(MailManager mm){
      // 构造委托实例，向事件登记回调方法
      mm.NewMail+=FaxMsg;
    }
    /// <summary>
    /// 回调方法
    /// </summary>
    /// <param name="sender">表示MailManager对象，便于将信息传递给他</param>
    /// <param name="e">表示MailManager对象想传给我们的附加信息</param>
    private void FaxMsg(object sender,NewMailEventArgs e){
      Console.WriteLine("msg:{0},{1},{2}",e.From,e.To,e.Subject);
    }

    /// <summary>
    /// 注销对事件的登记
    /// </summary>
    /// <param name="mm"></param>
    public void Unregister(MailManager mm){
      mm.NewMail-=FaxMsg;
    }
  }

static void Main(string[] args)
    {
      MailManager mm=new MailManager();
      Fax f=new Fax(mm);
      mm.SimulateNewMail("a","b","Hello World!");
      Console.ReadKey();
    }