两段小代码

private Action<int> sleep = second =>
           {
               Console.WriteLine("enter Action");
               Thread.Sleep(second * 1000);

               PrintThread();

               Console.WriteLine("leave Action");
           };

public void AsynT()
       {
           PrintThread();
           Console.WriteLine("begin");

           var result = sleep.BeginInvoke(2, null, null);

           Console.WriteLine("continue");

           sleep.EndInvoke(result);

           Console.WriteLine("end");
       }

       public void AsynOption()
       {
           PrintThread();
           var waiter = new ManualResetEvent(false);

           Console.WriteLine("begin");
           sleep.BeginInvoke(2, callback =>
           {
               //callback.AsyncWaitHandle.WaitOne();
               Console.WriteLine("end");
               sleep.EndInvoke(callback);
               waiter.Set();
           }, null);

           Console.WriteLine("continue");

           waiter.WaitOne();

           //Console.Read();
       }

在执行第二个方法的时候，我们可以会发现，如果去掉 ManualResetEvent 以后， callback代码不执行了。这是为什么呢？

经过仔细研究后，发现，

sleep.BeginInvoke(2, callback =>

相当于

sleep.BeginInvoke(2,new AsynCallback(…..)…..

使用回调方法。

从MSDN查到原因如下:

The callback is made on a ThreadPool thread. ThreadPool threads
are background threads, which do not keep the application running
if the main thread ends.