URL Unauthorized access error while accessing files from dropbox?

Hi This is Code for Generating Signature.

public static string GenerateSignature(Uri uri, string Method, string DropBox_AccessToken, string DropBox_AccessTokenSecret)
{
    var oAuth = new OAuthBase();

    var nonce = oAuth.GenerateNonce();
    var timeStamp = oAuth.GenerateTimeStamp();
    string parameters;
    string normalizedUrl;

    var signature = oAuth.GenerateSignature(uri, AppConstants.DropBoxConsumerKey, AppConstants.DropBoxConsumerSecret,
        DropBox_AccessToken, DropBox_AccessTokenSecret, Method, timeStamp, nonce,
        OAuthBase.SignatureTypes.HMACSHA1, out normalizedUrl, out parameters);

    //signature = HttpUtility.UrlEncode(signature);
    signature = WebUtility.UrlEncode(signature);

    StringBuilder requestUri = new StringBuilder(uri.ToString());
    requestUri.AppendFormat("?oauth_consumer_key={0}&", AppConstants.DropBoxConsumerKey);
    if (!string.IsNullOrEmpty(DropBox_AccessToken))
        requestUri.AppendFormat("oauth_token={0}&", DropBox_AccessToken);
    requestUri.AppendFormat("oauth_nonce={0}&", nonce);
    requestUri.AppendFormat("oauth_timestamp={0}&", timeStamp);
    requestUri.AppendFormat("oauth_signature_method={0}&", "HMAC-SHA1");
    requestUri.AppendFormat("oauth_version={0}&", "1.0");
    requestUri.AppendFormat("oauth_signature={0}", signature);

    return requestUri.ToString();
}