c#检查网络文件是否存在

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public  bool IsExist(string uri)       {           HttpWebRequest req = null;           HttpWebResponse res = null;           try           {               req = (HttpWebRequest)WebRequest.Create(uri);               req.Method = "HEAD";               req.Timeout = 100;               res = (HttpWebResponse)req.GetResponse();               return (res.StatusCode == HttpStatusCode.OK);           }           catch           {               return false;           }           finally           {               if (res != null)               {                   res.Close();                   res = null;               }               if (req != null)               {                   req.Abort();                   req = null;               }           }       }