获取文件长度（含网络)

        /// <summary>
        /// 获取文件长度，可为本地或网络
        /// </summary>
        /// <param name="url"></param>
        protected long GetFileLength(string url)
        {
            if (string.IsNullOrEmpty(url)) return 0;

            long length = 0;
            var request = WebRequest.Create(url);
            request.Credentials = CredentialCache.DefaultCredentials;
            if (request is HttpWebRequest)
            {
                var wr = (HttpWebRequest)request;
                wr.MaximumAutomaticRedirections = 5;
                wr.MaximumResponseHeadersLength = 10;
                var wre = wr.GetResponse();
                length = wre.ContentLength;
                wre.Close();
            }
            else if (request is FileWebRequest)
            {
                var fr = (FileWebRequest)request;
                var fre = fr.GetResponse();
                length = fre.ContentLength;
                fre.Close();
            }
            else
            {
                length = 0;
            }
            return length;
        }