DropNet,身份验证问题

我收到以下错误:"请求令牌未被用户正确授权"。问题在于RequestToAccessToken()方法。

下面是我的代码:

private DropNetClient _client;
private UserLogin _userlogin;
private string APP_KEY = "##";
private string APP_SECRET = "##";
private void Dropnet()
    {
        _client = new DropNetClient(APP_KEY, APP_SECRET);
        // Get request token asynchronously
        _client.GetTokenAsync(
            (userLogin) =>
            {
                // Authorize app
                var tokenUrl = _client.BuildAuthorizeUrl();
                RequestToAccessToken();
            },
            (error) =>
            {
                MessageBox.Show(error.Response.Content);
            });
    }
private void RequestToAccessToken()
    {
        _client.GetAccessTokenAsync(
               (userLogin) =>
               {
                   _userlogin = userLogin;

               },
               (error) =>
               {
                   MessageBox.Show(error.Response.Content);
               });
    }

任何帮助将不胜感激，谢谢!