static string fanhuiurl(string cahxunurl)
    {
        string url = "";
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(cahxunurl);
        req.Method = "HEAD";
        req.AllowAutoRedirect = false;
        HttpWebResponse myResp = (HttpWebResponse)req.GetResponse();
        if (myResp.StatusCode == HttpStatusCode.Redirect)
        { url = myResp.GetResponseHeader("Location"); }
        return url;
    }