Help!Help! this function GetAccessToken() return value always null.....

Question

VS2008 ASP.NET 2.0

Code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Net;
using System.IO;
using System.Runtime.Serialization.Json;
using System.Runtime.Serialization;
using System.Web;
using System.ServiceModel.Channels;
using System.ServiceModel;

namespace MicrosoftTranslatorSdk.HttpSamples
{
    class Program
    {
        static void Main(string[] args)
        {
            AdmAccessToken admToken;
            string headerValue;
            //Get Client Id and Client Secret from     https://datamarket.azure.com/developer/applications/  //Edit by RedCat 2012-5-18 in this page Get Client Id and Client Secret !!!!!
            //Refer obtaining AccessToken (http://msdn.microsoft.com/en-us/library/hh454950.aspx) 
            AdmAuthentication admAuth = new AdmAuthentication("clientID", "client secret");
            try
            {
                admToken = admAuth.GetAccessToken();
                // Create a header with the access_token property of the returned token
                headerValue = "Bearer " + admToken.access_token;
                DetectMethod(headerValue);

            }
            catch (WebException e)
            {
                ProcessWebException(e);
                Console.WriteLine("Press any key to continue...");
                Console.ReadKey(true);
            }
            catch (Exception ex)
            {

                Console.WriteLine(ex.Message);
                Console.WriteLine("Press any key to continue...");
                Console.ReadKey(true);
            }
        }

        private static void DetectMethod(string authToken)
        {
            Console.WriteLine("Enter Text to detect language:");
            string textToDetect = Console.ReadLine();
            //Keep appId parameter blank as we are sending access token in authorization header.
            string uri = "http://api.microsofttranslator.com/v2/Http.svc/Detect?text=" + textToDetect;
            HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create(uri);
            httpWebRequest.Headers.Add("Authorization", authToken);
            WebResponse response = null;
            try
            {
                response = httpWebRequest.GetResponse();
                using (Stream stream = response.GetResponseStream())
                {
                    System.Runtime.Serialization.DataContractSerializer dcs = new System.Runtime.Serialization.DataContractSerializer(Type.GetType("System.String"));
                    string languageDetected = (string)dcs.ReadObject(stream);
                    Console.WriteLine(string.Format("Language detected:{0}", languageDetected));
                    Console.WriteLine("Press any key to continue...");
                    Console.ReadKey(true);
                }
            }

            catch
            {
                throw;
            }
            finally
            {
                if (response != null)
                {
                    response.Close();
                    response = null;
                }
            }
        }
        private static void ProcessWebException(WebException e)
        {
            Console.WriteLine("{0}", e.ToString());
            // Obtain detailed error information
            string strResponse = string.Empty;
            using (HttpWebResponse response = (HttpWebResponse)e.Response)
            {
                using (Stream responseStream = response.GetResponseStream())
                {
                    using (StreamReader sr = new StreamReader(responseStream, System.Text.Encoding.ASCII))
                    {
                        strResponse = sr.ReadToEnd();
                    }
                }
            }
            Console.WriteLine("Http status code={0}, error message={1}", e.Status, strResponse);
        }
    }
    [DataContract]
    public class AdmAccessToken
    {
        [DataMember]
        private string _access_token;
        public string access_token
        {
            get { return _access_token; }
            set { _access_token = value; }
        }

        [DataMember]
        private string _token_type;
        public string token_type
        {
            get { return _token_type; }
            set { _token_type = value; }
        }

        [DataMember]
        private string _expires_in;
        public string expires_in
        {
            get { return _expires_in; }
            set { _expires_in = value; }
        }

        [DataMember]
        private string _scope;
        public string scope
        {
            get { return _scope; }
            set { _scope = value; }
        }

    }

    public class AdmAuthentication
    {
        public static readonly string DatamarketAccessUri = "https://datamarket.accesscontrol.windows.net/v2/OAuth2-13";
        private string clientId;
        private string cientSecret;
        private string request;

        public AdmAuthentication(string clientId, string clientSecret)
        {
            this.clientId = clientId;
            this.cientSecret = clientSecret;
            //If clientid or client secret has special characters, encode before sending request
            this.request = string.Format("grant_type=client_credentials&client_id={0}&client_secret={1}&scope=http://api.microsofttranslator.com", HttpUtility.UrlEncode(clientId), HttpUtility.UrlEncode(clientSecret));
        }

        public AdmAccessToken GetAccessToken()
        {
            return HttpPost(DatamarketAccessUri, this.request);
        }

        private AdmAccessToken HttpPost(string DatamarketAccessUri, string requestDetails)
        {
            //Prepare OAuth request 
            WebRequest webRequest = WebRequest.Create(DatamarketAccessUri);
            webRequest.ContentType = "application/x-www-form-urlencoded";
            webRequest.Method = "POST";
            byte[] bytes = Encoding.ASCII.GetBytes(requestDetails);
            webRequest.ContentLength = bytes.Length;
            using (Stream outputStream = webRequest.GetRequestStream())
            {
                outputStream.Write(bytes, 0, bytes.Length);
            }

            using (WebResponse webResponse = webRequest.GetResponse())
            {
                DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(AdmAccessToken));
                //Get deserialized object from JSON stream               
                AdmAccessToken token = (AdmAccessToken)serializer.ReadObject(webResponse.GetResponseStream());
                return token;
            }


            return null;


        }
    }
}

-------------------------------------------------------------------------

aspx Page_Load Code:

        AdmAccessToken admToken;
        string headerValue;
        //Get Client Id and Client Secret from https://datamarket.azure.com/developer/applications/
        AdmAuthentication admAuth = new AdmAuthentication("45325432453543", "eb5WLTSpFDgigkDBhlkjgsvuS3jDkRCcK");

        admToken = admAuth.GetAccessToken();
        // Create a header with the access_token property of the returned token
        headerValue = "Bearer" + " " + HttpUtility.UrlEncode(admToken.access_token);

-----------------------------------------------------------------------------------------------------------------------------

Question:

clientId,clientSecret No problem.but 

admToken = admAuth.GetAccessToken();
admToken.access_token

Always Return Null. I have a headache, do not know where the problem.

Urgent, please help me！