.net异步回调

Code
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;

namespace Async2
{
    public class MyAsync
    {
        public delegate string LengthyProcedureAsyncStub(int millisenconds);
        public string LengthyProcedure(int milliseconds)
        {
            System.Threading.Thread.Sleep(milliseconds);
            return "success";
        }
        public IAsyncResult BeginLengthyProcedure(int millseconds, AsyncCallback callback)
        {
            LengthyProcedureAsyncStub stub = new LengthyProcedureAsyncStub(LengthyProcedure);
            IAsyncResult result = stub.BeginInvoke(millseconds, callback, stub);
            return result;
        }
        public string EndLengthyProcedure(IAsyncResult result)
        {
            LengthyProcedureAsyncStub stub = (LengthyProcedureAsyncStub)result.AsyncState;
            string res = stub.EndInvoke(result);
            return res;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            MyAsync ma = new MyAsync();
            IAsyncResult result = ma.BeginLengthyProcedure(5000, new AsyncCallback(showmessage));
            Console.WriteLine(ma.EndLengthyProcedure(result));
            Console.WriteLine("main");
            Console.Read();
        }
        static void showmessage(IAsyncResult result)
        {
            Console.WriteLine("showmessage");
        }
    }
}