浅谈AsyncState与AsyncDelegate使用的异同

对于AsyncState来说，其MSDN的解释为：得到BeginInvoke方法的最后一个参数。而对于AsyncDelegate来说，其MSDN的解释为：得到异步调用的委托对象。也就是异步调用的委托源。

对于委托的异步调用来说，其BeginInvoke函数无非包括以下内容,BeginInvoke(调用参数，回调函数，Object对象)

如果想利用AsyncState来还原对象的话，这里的Object对象必须是源委托；如果利用AsyncDelegate的话，这里可以为空，可以为源委托。具体区别请看下面的例子：

//AsyncState方式还原委托对象
chatDelegate.BeginInvoke(this, e, new AsyncCallback((iar) =>
{

     ChatDelegate thisDelegate = (ChatDelegate)iar.AsyncState;
     thisDelegate.EndInvoke(iar);
}), chatDelegate);

 //AsyncDelegate方式还原委托对象
chatDelegate.BeginInvoke(this, e, new AsyncCallback((iar) =>
{
     AsyncResult ar = (AsyncResult)iar;
     ChatDelegate thisDelegate = (ChatDelegate)ar.AsyncDelegate;
     thisDelegate.EndInvoke(iar);
}), null);

可以看到，当利用AsyncState时候，最后一个对象必须为源委托；当利用AsyncDelegate的时候，最后一个对象可以为null.

全部代码如下：

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private delegate void ChatDelegate(object sender, MyEventArgs e);
        private static event ChatDelegate ChatEvent;

        private void Form1_Load(object sender, EventArgs e)
        {
            MyEventArgs myEventArgs = new MyEventArgs();
            myEventArgs.Message = "this is test args";
           
            ChatEvent+=new ChatDelegate(Form1_ChatEvent);

            ThrowEvent(myEventArgs);
        }

        private void Form1_ChatEvent(object sender, MyEventArgs e)
        {
            MessageBox.Show(e.Message);
        }

        private void ThrowEvent(MyEventArgs e)
        {
            ChatDelegate tempDelegate = ChatEvent;
            if(tempDelegate != null)
            {
                foreach (ChatDelegate chatDelegate in tempDelegate.GetInvocationList())
                {
                    //AsyncState方式还原委托对象
                    chatDelegate.BeginInvoke(this, e, new AsyncCallback((iar) =>
                    {
                        ChatDelegate thisDelegate = (ChatDelegate)iar.AsyncState;
                        thisDelegate.EndInvoke(iar);
                    }), chatDelegate);

                    //AsyncDelegate方式还原委托对象
                    //chatDelegate.BeginInvoke(this, e, new AsyncCallback((iar) =>
                    //{
                    //    AsyncResult ar = (AsyncResult)iar;
                    //    ChatDelegate thisDelegate = (ChatDelegate)ar.AsyncDelegate;
                    //    thisDelegate.EndInvoke(iar);
                    //}), null);
                }
            }
        }
    }

    public class MyEventArgs : EventArgs
    {
        public string Message { get; set; }
    }
}