异步回调时在调用 BeginInvoke时提供的回调方法，主线程就不必再等待异步线程工作完毕，异步线程在工作结束后会主动调用提供的回调方法。

    class Program

    {

        public delegate void PrintDelegate(string content);

        static void Main(string[] args)

        {

            int threadId = Thread.CurrentThread.ManagedThreadId;

            PrintDelegate printDelegate = new PrintDelegate(Print);

            Console.WriteLine("[主线程id:{0}]\t开始调用打印的方法...", threadId);

            IAsyncResult result = printDelegate.BeginInvoke("Hello Word!", PrintComplete,printDelegate);

            Thread.Sleep(1000);

            Console.ReadLine();

        }

        public static void Print(string content)

        {

            int threadId = Thread.CurrentThread.ManagedThreadId;

            Console.WriteLine("[当前线程id:{0}]\t{1}",threadId,content);

            Thread.Sleep(1000);

        }

        private static void PrintComplete(IAsyncResult asyResult)

        {

            if (null == asyResult)

            {

                throw new ArgumentException();

            }

            int threadId = Thread.CurrentThread.ManagedThreadId;

            (asyResult.AsyncState as PrintDelegate).EndInvoke(asyResult);

            Console.WriteLine("[当前线程id:{0}]\t打印方法调用完毕。", threadId);

        }

    }

基于委托的异步回调