flickr oauth签名无效错误

我使用OauthBase.cs类生成签名，但经常收到一个错误，说签名无效。有人能告诉我哪里出了问题吗？

我暂时将其显示在消息框中，并将其复制粘贴到浏览器中以进行查看。

这是我的代码。。

        string consumerKey = "consumer_key";
        string consumerSecret = "consumer_secret";
        string normalizedUrl = null;
        string normalizedRequestParameters = null;

        Uri url = new Uri("http://www.flickr.com/services/oauth/request_token");
        oAuthBase oAuth = new oAuthBase();
        string nonce = oAuth.GenerateNonce();
        string timeStamp = oAuth.GenerateTimeStamp();
        string callback = oAuth.UrlEncode("http://www.example.com");
        string sig = oAuth.GenerateSignature(url, consumerKey, consumerSecret, string.Empty   , string.Empty , "GET", timeStamp, nonce, callback , oAuthBase.SignatureTypes.HMACSHA1,out normalizedUrl ,out normalizedRequestParameters );
        sig = HttpUtility.UrlEncode(sig);
        StringBuilder sb = new StringBuilder(url.ToString());

        sb.AppendFormat("?oauth_nonce={0}&", nonce);
        sb.AppendFormat("oauth_timestamp={0}&", timeStamp);
        sb.AppendFormat("oauth_consumer_key={0}&", consumerKey);
        sb.AppendFormat("oauth_signature_method={0}&", "HMAC-SHA1");           
        sb.AppendFormat("oauth_version={0}&", "1.0");
        sb.AppendFormat("oauth_signature={0}", sig);
        sb.AppendFormat("oauth_callback={0}", callback);

        MessageBox.Show(sb.ToString());