c# 判断远程文件是否存在

//1:
public static 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;
                }
            }
        } 

//2:

private bool UrlExistsUsingXmlHttp(string url)
{
  //注意：此方法需要引用Msxml2.dll
  MSXML2.XMLHTTP _xmlhttp = new MSXML2.XMLHTTPClass();
  _xmlhttp.open("HEAD", url, false, null, null);
  _xmlhttp.send("");
  return (_xmlhttp.status == 200);
}

//3:
private bool UrlExistsUsingSockets(string url)
{
  if (url.StartsWith("http://")) url = url.Remove(0, "http://".Length);
  try
  {
    System.Net.IPHostEntry ipHost =System.Net.Dns.GetHostEntry(url);// System.Net.Dns.Resolve(url);
    return true;
  }
  catch (System.Net.Sockets.SocketException se)
  {
    System.Diagnostics.Trace.Write(se.Message);
    return false;
  }
}