调用云解析API，总是报SignatureDoesNotMatch错误，求解决

我调用云解析老是报 SignatureDoesNotMatch错误，求解决，程序语言是C#，代码如下：



public class AliyunDNSUtil
    {
        public static void UpdateDNS(string ip)
        {
            var x = GetDomainList();
        }


        private static string UrlEncoder(string value)
        {
            return HttpUtility.UrlEncode(value).Replace("+", "%20").Replace("*", "%2A").Replace("%7E", "~");
        }


        private static SortedDictionary<string, string> _PublicArguments;
        private static SortedDictionary<string, string> PublicArguments
        {
            get
            {
                if (_PublicArguments == null)
                {
                    var result = new SortedDictionary<string, string>();


                    result.Add("Format", UrlEncoder("XML"));
                    result.Add("Version", UrlEncoder("2015-01-09"));
                    result.Add("AccessKeyId", UrlEncoder(Settings.AliyunAccessKeyID));
                    result.Add("SignatureMethod", UrlEncoder("HMAC-SHA1"));
                    result.Add("Timestamp", UrlEncoder(DateTime.UtcNow.ToString("yyyy-MM-ddTHH:mm:ssZ")));
                    result.Add("SignatureVersion", UrlEncoder("1.0"));
                    result.Add("SignatureNonce", UrlEncoder(Guid.NewGuid().ToString("D")));


                    _PublicArguments = result;
                }
                return _PublicArguments;
            }
        }


        private static string StringToSign(string value)
        {
            Encoding encode = Encoding.UTF8;
            byte[] byteData = encode.GetBytes(value);
            byte[] byteKey = encode.GetBytes(Settings.AliyunAccessKeySecret + "&");
            HMACSHA1 hmac = new HMACSHA1(byteKey);
            CryptoStream cs = new CryptoStream(Stream.Null, hmac, CryptoStreamMode.Write);
            cs.Write(byteData, 0, byteData.Length);
            cs.Close();
            return Convert.ToBase64String(hmac.Hash);
        }


        private static XDocument GetDomainList()
        {
            var arguments = new SortedDictionary<string, string>();
            foreach (var item in PublicArguments.Keys)
            {
                arguments.Add(item, PublicArguments[item]);
            }
            arguments.Add("Action", UrlEncoder("DescribeDomains"));
            arguments.Add("KeyWord", UrlEncoder(Settings.DomainName));


            var argumentssb = new StringBuilder();
            foreach (var item in arguments.Keys)
            {
                argumentssb.Append("&" + item + "=" + arguments[item]);
            }


            var argumentsstr = argumentssb.ToString().Substring(1);
            var signstring = "GET&%2F&" + UrlEncoder(argumentsstr);
            var encodedargs = argumentsstr + "&Signature=" + UrlEncoder(StringToSign(signstring));
            var url = "https://dns.aliyuncs.com/?" + encodedargs;
            var xmlstream = HttpRequest(url);
            if (xmlstream == null)
            {
                return null;
            }
            var reader = new StreamReader(xmlstream, Encoding.UTF8);
            var str = reader.ReadToEnd();


            return XDocument.Load(xmlstream);
        }


        private static Stream HttpRequest(string url)
        {
            try
            {
                var httpReq = (HttpWebRequest)WebRequest.Create(url);
                var httpResp = (HttpWebResponse)httpReq.GetResponse();
                return httpResp.GetResponseStream();
            }
            catch (Exception)
            {
                return null;
            }
        }
    }