アマゾンウェブサービスのGetAuthTokenメソッドが機能しない

GetAuthToken（）メソッドでアマゾンウェブサービスにアクセスしようとしています。しかし、署名を取得するたびにエラーが修正されません。このエラーが発生するたびに。正確な理由が見つかりません

誰かがこれの実用的な例を共有したり、私がここで何をしているのかを見ることができますか？

私のコード：

 private void GetToken()
{
    string secretKey = "my secret key";       
    string serviceUrl = "https://mws.amazonservices.com/Sellers/2011-07-01";
    string CHARACTER_ENCODING = "UTF-8";
    string ALGORITHM = "HmacSHA256";

    // Create set of parameters needed and store in a map
    Dictionary<string, string> parameters = new Dictionary<string,string>();

    // Add required parameters. Change these as needed.
    parameters.Add("AWSAccessKeyId", "my access id");
    parameters.Add("Action","GetAuthToken");
    parameters.Add("MWSAuthToken", "");
    parameters.Add("SellerId","my seller id");
    parameters.Add("SignatureMethod", ALGORITHM);
    parameters.Add("SignatureVersion",("2");

    parameters.Add("Timestamp", "2015-07-06T16:03:00Z");
    parameters.Add("Version", "2011-07-01");

    // Format the parameters (without the signature parameter)
    string formattedParameters = calculateStringToSignV2(parameters, serviceUrl);

    string signature = signNew(formattedParameters, secretKey);

    // Add signature to the parameters 
    parameters.Add("Signature", signature);

    formattedParameters = calculateStringToSignV2(parameters, serviceUrl);



    string querystring = formattedParameters.Substring(formattedParameters.IndexOf("AWSAccessKeyId"));


      // Setup the HTTP request.
    HttpWebRequest objWebRequest = (HttpWebRequest)WebRequest.Create(serviceUrl + "?" + querystring);
    objWebRequest.Method = "POST";
    objWebRequest.AllowAutoRedirect = true;
    objWebRequest.ContentLength = querystring.Length;

    // Post to the login form.
    StreamWriter swRequestWriter = new StreamWriter(objWebRequest.GetRequestStream());
    swRequestWriter.Write(querystring);
    swRequestWriter.Close();
    HttpWebResponse objWebResponse = null;
    try
    {
        // Get the response.
        objWebResponse =
            (HttpWebResponse)objWebRequest.GetResponse();
    }
    catch(WebException e)
    {

    }
    // Read the response
    StreamReader srResponseReader = new
        StreamReader(objWebResponse.GetResponseStream());
    string strResponseData = srResponseReader.ReadToEnd();
    srResponseReader.Close();

    responseTxt.Text =strResponseData;



private static string calculateStringToSignV2(Dictionary<string, string> _Parameters, string serviceUrl)
{


    Uri endpoint = new Uri(serviceUrl.ToLower());
    StringBuilder data = new StringBuilder();
    data.Append("POST\n");
    data.Append(endpoint.Host);
    data.Append("\n/");
    data.Append("\n");

    SortedDictionary<string, string> sorted = new SortedDictionary<string, string>();
    foreach (KeyValuePair<String, String> pair in _Parameters)
    {
        sorted.Add(pair.Key, pair.Value);
    }


    if (sorted.Count > 0)
    {
        //data.Append("?");
        bool first = true;
        foreach (KeyValuePair<String, String> pair in sorted)
        {
            if (!first)
            {
                data.Append("&");
            }
            else
            {
                first = false;
            }
            data.Append((pair.Key));
            data.Append("=");
            data.Append((pair.Value));
        }
    }

    return data.ToString();
}

}

  private static String signNew(string parameters, string secretKey)
     {           

         string data = parameters;

         HMACSHA256 hmac = new    HMACSHA256(Encoding.UTF8.GetBytes(secretKey));
         hmac.Initialize();

         byte[] bytes = Encoding.UTF8.GetBytes(data);
         byte[] signature = hmac.ComputeHash(bytes);

         string signatureBase64 = Convert.ToBase64String(signature);

         return signatureBase64;           
  }

  private static String urlEncodeNew(String rawValue) {

    string value = (rawValue == null) ? "" : rawValue;
    string encoded = null;

    try {
        encoded = System.Web.HttpUtility.UrlEncode(rawValue, Encoding.UTF8).Replace("+", "%20")
            .Replace("*", "%2A")
            .Replace("%7E", "~");


    } catch (Exception e) {

    }

    return encoded;
}