facebook c#sdk-用户已经'；t授权应用程序执行此操作

我正在构建一个控制台应用程序，它将把流发布到页面的墙上。

问题：我收到"用户未授权应用程序执行此操作"。我正在使用opengraph获取访问令牌。

我是不是错过了什么？非常感谢您的帮助。谢谢

// constants
string apiKey = "XXX";
string secret = "XXX";
string pageId = "XXX";
// get access token
string url = string.Format("https://graph.facebook.com/oauth/access_token?client_id={0}&client_secret={1}&grant_type=client_credentials", apiKey, secret); // todo: figure out open graph url
WebRequest req = WebRequest.Create(url);
WebResponse resp = req.GetResponse();
StreamReader reader = new StreamReader(resp.GetResponseStream());
string respStr = reader.ReadToEnd();
string accessToken = respStr.Replace("access_token=", "");
// construct the post
dynamic messagePost = new ExpandoObject();
messagePost.access_token = accessToken;
messagePost.picture = "www.google.com/pic.png";
messagePost.link = "www.google.com";
messagePost.name = "some name";
messagePost.captiion = "some caption";
messagePost.description = "some description";
messagePost.req_perms = "publish_stream";
messagePost.scope = "publish_stream";
// using client
FacebookClient client = new FacebookClient(accessToken);
try // to post the post to the page's wall
{
    var result = client.Post(string.Format("/{0}/feed", pageId), messagePost); 
}
    catch (FacebookOAuthException ex)
{
    // getting caught here, with error msg = "The user hasn't authorized the application to perform this action"
}
catch (FacebookApiException ex)
{
    // ignore
}