如何在 C# 中终止 HttpWebRequest 连接？即使设置超时或读写超时也不起作用

Question

我想终止一个httpwebrequest，当它需要太长时间的连接。下面是我写的一段简单的代码:

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);
            request.Timeout = 5000;
            request.ReadWriteTimeout = 5000;
            request.Proxy = new WebProxy("http://" + proxyUsed + "/", true);
            request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.01; Windows NT 5.0)";
            using (WebResponse myResponse = request.GetResponse())
            {                    
                using (Stream s = myResponse.GetResponseStream())
                {
                    s.ReadTimeout = 5000;
                    s.WriteTimeout = 5000;
                    using (StreamReader sr = new StreamReader(s, System.Text.Encoding.UTF8))
                    {                            
                        result = sr.ReadToEnd();
                        httpLink = myResponse.ResponseUri.AbsoluteUri;
                        sr.Close();
                    }
                    s.Close();
                }
                myResponse.Close();
            }

然而，有时连接需要大约15分钟才能获得响应。情况是15分钟后，我仍然可以得到响应，但不是URL的完整源代码。我猜连接太慢了，URL在超时时间内响应我一点数据，只是说例如在5秒内接收1字节，所以它不会过期，但它很长。如何终止连接?谢谢:)

Answer 1

您可能会发现超时实际上是有效的，但是线程在试图关闭流时挂起。我不知道为什么会这样，但有时候就是这样。我从未使用过ReadToEnd，但我在使用Read时遇到过这种情况。

在我关闭流之前，我通过在请求上调用Abort来修复这个问题。它有点笨重，但很有效。下面的简短代码展示了该技术。

HttpWebResponse response = null;
StreamReader sr = null;
try
{
    response = (HttpWebResponse)request.GetResponse(...);
    Stream s = response.GetResponseStream();
    sr = new StreamReader(s, Encoding.UTF8);
    // do your reading here
}
finally
{
    request.Abort();  // !! Yes, abort the request
    if (sr != null)
        sr.Dispose();
    if (response != null)
        response.Close();
}

我发现ReadTimeout和ReadWriteTimeout工作如预期。也就是说，当读取超时时，执行确实转到finally块。如果request.Abort不在那里，对sr.Dispose的调用将挂起。

Answer 2

中断流读取，如果总时间太长则中止。

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);
request.Timeout = 5000;
request.ReadWriteTimeout = 5000;
request.Proxy = new WebProxy("http://" + proxyUsed + "/", true);
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.01; Windows NT 5.0)";//ahem! :)
DateTime giveUp = DateTime.UtcNow.AddSeconds(5); 
using (WebResponse myResponse = request.GetResponse())
{                    
    httpLink = myResponse.ResponseUri.AbsoluteUri;
    using (Stream s = myResponse.GetResponseStream())
    {
        s.ReadTimeout = 5000;
        s.WriteTimeout = 5000;
        char[] buffer = new char[4096];
        StringBuilder sb = new StringBuilder()
        using (StreamReader sr = new StreamReader(s, System.Text.Encoding.UTF8))
        {                            
            for(int read = sr.Read(buffer, 0, 4096); read != 0; read = sr.Read(buffer, 0, 4096))
            {
                if(DateTime.UtcNow > giveUp)
                    throw new TimeoutException();
                sb.Append(buffer, 0, read);
            }
            result = sb.ToString();
        }
    }
}

Answer 3

确保在异常情况下也关闭连接。在这种情况下，WebResponse对象位于WebException:

try
{
    using (WebResponse myResponse = request.GetResponse())
      // do stuff
}
catch (WebException webEx)
{
    webEx.Response.Close();
}