为什么我的这个访问url的函数总是超时

C# code

 public void ToURL(string url)
        {
            WebRequest req = WebRequest.Create(url);
            WebResponse res = req.GetResponse();
            res = null;
            req = null;
        }

<exception>
  <Time>2008-2-26 12:29:20</Time>
  <Message>操作超时</Message>
  <Source>System
  </Source>
  <StackTrace> 在 System.Net.HttpWebRequest.GetResponse()

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