异步回调反回直

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
namespace sample
{
    class AsyncDemo
    {
        public string TestMethod(int callDuration, out int threadid)
        {
            Console.WriteLine("Test Method begins");
            Thread.Sleep(callDuration);
            threadid = System.Threading.Thread.CurrentThread.ManagedThreadId;
            return "MyCallTime was" + callDuration.ToString();
        }
    }
    public delegate string AsyncDelegate(int callDuration, out int threadid);
    class Program
    {
        static void Main(string[] args)
        {
            int threadID;
            AsyncDemo ad = new AsyncDemo();
            AsyncDelegate andl = new AsyncDelegate(ad.TestMethod);
            IAsyncResult ar = andl.BeginInvoke(3000, out threadID,
                new AsyncCallback(CallBackMethod), andl);
            Thread.Sleep(10);
            Console.WriteLine("Main Thread {0} Does Some Work",
               System.Threading.Thread.CurrentThread.ManagedThreadId);

            Console.ReadLine();
        }

        static void CallBackMethod(IAsyncResult ar) //无返回而且只有一个IAsyncResult类型的参数ar
        {
            int j;
            AsyncDelegate andl = (AsyncDelegate)ar.AsyncState;
            string a = andl.EndInvoke(out j, ar);//这个是返回值关键
            Console.WriteLine(a);
        }
    }
}