获取下载文件长度

    private long getDownLength(string url)
    {
        long length = 0;
        try
        {
            var req = (HttpWebRequest)WebRequest.CreateDefault(new Uri(url));
            req.Method = "HEAD";
            req.Timeout = 5000;
            var res = (HttpWebResponse)req.GetResponse();
            if (res.StatusCode == HttpStatusCode.OK)
            {
                length = res.ContentLength;
            }
            res.Close();
            return length;
        }
        catch (Exception)
        {
            return 0;
            throw;
        }
    }