根据上面的讨论，我们可以如下设计我们的Asynchronous Http Hanlder：

public class AsyncFakePHPHttpHandler : IHttpAsyncHandler 
{ 
private void ProcessRequestCallback(object state) 
{ 
AsyncResult async = (AsyncResult)state; 
// this is where the real work is done 
ProcessRequest(async.context); 
async.isCompleted = true; 
if (null != async.asyncCallback) 
async.asyncCallback(async); 
} 
public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback asyncCallback, object state) 
{ 
AsyncResult async = new AsyncResult(context, asyncCallback, state); 
// if the callback is null, we can return immediately and let EndProcessRequest do all the job 
// if callback is not null, we will use our thread pool to execute the necessary asynchronous operations 
// what happens in ASP.NET is that the callback in NOT null, so QueueUserWorkItem will be used 
if (null != async.asyncCallback) 
threadPool.QueueUserWorkItem(ProcessRequestCallback, async); 
return async; 
} 
// this design also satisfies method 4), we implement it this way to follow the Asynchronous Pattern as much as we can 
public void EndProcessRequest(IAsyncResult result) 
{ 
AsyncResult async = (AsyncResult)result; 
if (null == async.asyncCallback) 
ProcessRequest(async.context); 
} 
}

   总结一下实现异步Http Handler的要点：

   1) 所有在BeginProcessRequest中的耗时操作（比如IO什么的）都应该采用异步调用（比如BeingRead/EndRead）或者生成新的线程去执行。不错，你可以设计一个blocking BeginProcessRequest，没有人能阻止你这么做。But that's a BAD BAD idea.

   2) 实现BeginProcessRequest/EndProcessRequest的目的是允许ASP.NET来异步调用你的Http Handler

   3) 你应该创建一个实现IAsyncResult接口的类，在BeginProcessRequest中你会生成一个该类的实例并返回给ASP.NET（注意BeginProcessRequest的返回值类型）。而根据Asynchronous Pattern，ASP.NET在调用EndProcessRequest的时候会把这个实例再传回给你，你可以用这个实例来判断你所执行的任务的当前状态。