通过异步处理程序构建的长连接消息推送机制，单台服务器可以轻松支持上万个并发连接

public class Handler : IHttpHandler, IHttpAsyncHandler
{
    public void ProcessRequest(HttpContext context)
    {
        //异步处理器不执行该方法
    }

    public bool IsReusable
    {
        //设置允许重用对象
        get { return false; }
    }
    
    //请求开始时由ASP.NET调用此方法
    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
        context.Response.ContentType = "text/xml";
        context.Response.Write("App:");
        context.Response.Write(context.ApplicationInstance.GetHashCode());
        context.Response.Write("\tBegin:");
        context.Response.Write(DateTime.Now.ToString("mm:ss,ffff"));
        //输出当前线程
        context.Response.Write("\tThreadId:");
        context.Response.Write(Thread.CurrentThread.ManagedThreadId);
        //构建异步结果并返回
        var result = new WebAsyncResult(cb, context);
        //用一个定时器来模拟异步触发完成
        Timer timer = null;
        timer = new Timer(o =>
        {
            result.SetComplete();
            timer.Dispose();
        }, null, TimeSpan.FromSeconds(5), TimeSpan.FromSeconds(5));
        return result;
    }

    //异步结束时，由ASP.NET调用此方法
    public void EndProcessRequest(IAsyncResult result)
    {
        WebAsyncResult webresult = (WebAsyncResult)result;
        webresult.Context.Response.Write("\tEnd:");
        webresult.Context.Response.Write(DateTime.Now.ToString("mm:ss,ffff"));
        //输出当前线程
        webresult.Context.Response.Write("\tThreadId:");
        webresult.Context.Response.Write(Thread.CurrentThread.ManagedThreadId);
    }

    //WEB异步方法结果
    class WebAsyncResult : IAsyncResult
    {
        private AsyncCallback _callback;

        public WebAsyncResult(AsyncCallback cb, HttpContext context)
        {
            Context = context;
            _callback = cb;
        }

        //当异步完成时调用该方法
        public void SetComplete()
        {
            IsCompleted = true;
            if (_callback != null)
            {
                _callback(this);
            }
        }

        public HttpContext Context
        {
            get;
            private set;
        }

        public object AsyncState
        {
            get { return null; }
        }

        //由于ASP.NET不会等待WEB异步方法，所以不使用此对象
        public WaitHandle AsyncWaitHandle
        {
            get { throw new NotImplementedException(); }
        }

        public bool CompletedSynchronously
        {
            get { return false; }
        }

        public bool IsCompleted
        {
            get;
            private set;
        }
    }
}