Unable to Post to user's Facebook Timeline via "publish_actions" Permission - asp.net

Question

I'm building a small website in asp.net which let the user login via facebook and then the app
can post to user's timeline, i'm using scope="publish_actions,manage_pages" but when user login, app never asks the permission to post on the wall, it only gets the permission to access public profile, here is my code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Web;
using System.Web.Mvc;
using System.IO;
using Facebook;
namespace EadProject.Controllers
{
public class LoginController : Controller
{
//
// GET: /Login/
public ActionResult Index()
{
return View();
}
public ActionResult FacebookLogin()
{
string app_id = "1415837275360274";
string app_secret = "911b08ff5ae3cdaf74e512ce1c312c65";
string scope = "publish_actions,manage_pages";
if (Request["code"] == null)
{
Response.Redirect(string.Format("https://graph.facebook.com/oauth/authorize?client_id={0}&redirect_uri={1}&scope={2}",app_id,Request.Url.AbsoluteUri,scope));
}
else
{
Dictionary<string, string> tokens = new Dictionary<string, string>();
string url = string.Format("https://graph.facebook.com/oauth/access_token?client_id={0}&redirect_uri={1}&scope={2}&code={3}&client_secret={4}", app_id, Request.Url.AbsoluteUri, scope,Request["code"].ToString(),app_secret);
HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
{
StreamReader reader = new StreamReader(response.GetResponseStream());
string vals = reader.ReadToEnd();
foreach (string token in vals.Split('&'))
{
tokens.Add(token.Substring(0,token.IndexOf("=")),token.Substring(token.IndexOf("=")+1,token.Length-token.IndexOf("=")-1));
}
}
string access_token = tokens["access_token"];
var client = new FacebookClient(access_token);
client.Post("/me/feed", new { message = "I am using QUiz Contest Pucit App developed by AHmed ALI :)" });
StreamWriter sw = new StreamWriter(new FileStream("C:\tokens.txt",FileMode.OpenOrCreate));
sw.WriteLine(access_token);
}
return View();
}
}
}
Please explain what is the problem?