C# 网络地址是否有效

 /// <summary>
        /// 判断链接是否有效
        /// </summary>
        /// <param name="url">url</param>
        /// <returns></returns>
        private bool UrlIsExist(string url)
        {
            System.Uri u = null;
            try
            {
                u = new Uri(url);
            }
            catch { return false; }
            bool isExist = false;
            System.Net.HttpWebRequest r = System.Net.HttpWebRequest.Create(u) as System.Net.HttpWebRequest;
            r.Method = "HEAD";
            try
            {
                System.Net.HttpWebResponse s = r.GetResponse() as System.Net.HttpWebResponse;
                if (s.StatusCode == System.Net.HttpStatusCode.OK)
                {
                    isExist = true;
                }
            }
            catch (System.Net.WebException x)
            {
                try
                {
                    isExist = ((x.Response as System.Net.HttpWebResponse).StatusCode != System.Net.HttpStatusCode.NotFound);
                }
                catch { isExist = (x.Status == System.Net.WebExceptionStatus.Success); }
            }
            return isExist;
        }