本来的代码大抵如下：

private void foo(string url) { using (WebClient client = new WebClient()) { client.DownloadProgressChanged += (obj, ev) => { client.CancelAsync(); }; client.DownloadStringCompleted += (obj, ev) => { ................ Thread.Sleep(30000); bar(client, newUrl); } } } private void bar(WebClient client, string url) { client.DownloadStringAsync(url); }

原来以为这样可以自动访谒一串网址(这里省略了取下一个url的代码)，但运行后发明，经常会运行到一半停下来，措施并没有死失，只是遏制了，不继续访谒下一个网址。

开始以为是Thread.Sleep()在做怪，把它移到bar要领里：

private void bar(WebClient client, string url) { Thread.Sleep(30000); client.DownloadStringAsync(url); }

功效没用。想了半天，最后预计是访谒某个网址时，一直在请求，，但一直充公到数据，所以没有触发DownloadProgressChanged 事件。又因为是异步执行，所以UI没有梗阻。

最后用了个Timer控件，把代码改成：

private void foo(string url) { using (WebClient client = new WebClient()) { client.DownloadProgressChanged += (obj, ev) => { client.CancelAsync(); }; client.DownloadStringCompleted += (obj, ev) => { ................ if (timer1.Enabled) { timer1.Stop(); } bar(client, newUrl); } timer1.Tick += (obj, ev) => { timer1.Stop(); if (client.IsBusy) { client.CancelAsync(); } else { bar(client, url); } } bar(client, url); } } private void bar(WebClient client, string url) { Thread.Sleep(30000); client.DownloadStringAsync(url); timer1.Stop(); timer1.Start(); }

Timer的间隔也设成30秒。测试了几百个url没有问题。

浏览器自动化的一些体会11 webclient的异步操纵