原来的代码大致如下：
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);
}
本来以为这样可以自动访问一串网址(这里省略了取下一个url的代码)，但运行后发现，经常会运行到一半停下来，程序并没有死掉，只是停止了，不继续访问下一个网址。
开始以为是Thread.Sleep()在做怪，把它移到bar方法里：
private void bar(WebClient client, string url)
{
Thread.Sleep(30000);
client.DownloadStringAsync(url);
}
结果没用。想了半天，最后估计是访问某个网址时，一直在请求，但一直没收到数据，所以没有触发DownloadProgressChanged 事件。又因为是异步执行，所以UI没有阻塞。
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。
原文：https://www.cnblogs.com/badnumber/p/12216522.html