Please help with Google AOuth authentification in C#. Fails to exchange authorised request token for an access token.

Question

The code fails to "Upgrade to Access Token" but fully matches the AOuth Playground requests.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Security.Cryptography;
using System.Net;
namespace TestGAOuth
{
    class Program
    {
        const string OAuthGetRequestToken = "https://www.google.com/accounts/OAuthGetRequestToken";
        const string OAuthAuthorizeToken = "https://www.google.com/accounts/OAuthAuthorizeToken";
        const string OAuthGetAccessToken = "https://www.google.com/accounts/OAuthGetAccessToken";
        static void Main(string[] args)
        {
            //1) Choose your scope:
            string scope = "https://www.google.com/analytics/feeds/";
            //2)Modify the OAuth Parameters:
            string oauth_consumer_key = "anonymous";
            string consumerSecret = "anonymous";
            //3)Get a Request Token:
            string timestamp = GenerateTimeStamp();
            string nonce = GenerateNonce();

            //Signature:
            StringBuilder sb = new StringBuilder();
            sb.AppendFormat("oauth_callback={0}", "oob");
            sb.AppendFormat("&oauth_consumer_key={0}", oauth_consumer_key);
            sb.AppendFormat("&oauth_nonce={0}", nonce);
            sb.Append("&oauth_signature_method=HMAC-SHA1");
            sb.AppendFormat("&oauth_timestamp={0}", timestamp);
            sb.Append("&oauth_version=1.0");
            sb.AppendFormat("&scope={0}", UrlEncode(scope));
            StringBuilder signatureBase = new StringBuilder("GET");
            signatureBase.AppendFormat("&{0}", UrlEncode(OAuthGetRequestToken));
            signatureBase.AppendFormat("&{0}", UrlEncode(sb.ToString()));
            string sign = signatureBase.ToString();
            Debug.WriteLine("Signature Base 1:" + sign);
            string key = string.Format("{0}&", UrlEncode(consumerSecret));
            HMACSHA1 hmacsha1 = new HMACSHA1();
            hmacsha1.Key = Encoding.UTF8.GetBytes(key);
            byte[] dataBuffer = System.Text.Encoding.UTF8.GetBytes(sign);
            byte[] hashBytes = hmacsha1.ComputeHash(dataBuffer);
            string signature = Convert.ToBase64String(hashBytes);
            string header1 = string.Format("OAuth oauth_consumer_key=\"{0}\",oauth_callback=\"oob\",oauth_nonce=\"{1}\",oauth_signature_method=\"HMAC-SHA1\",oauth_timestamp=\"{2}\",oauth_version=\"1.0\",oauth_signature=\"{3}\"",
                UrlEncode(oauth_consumer_key), UrlEncode(nonce), UrlEncode(timestamp), UrlEncode(signature));
            string request1 = OAuthGetRequestToken + string.Format("?scope={0}", UrlEncode(scope));
            WebClient wc = new WebClient();
            wc.Headers["Authorization"] = header1;
            string result = wc.DownloadString(request1);
            Debug.Write(result);
            string[] pairs = result.Split('&');
            string oauth_token = pairs[0].Substring(12);
            string oauth_token_secret = pairs[1].Substring(19);
            //4)Authorize the Request Token:
            string request2 = OAuthAuthorizeToken + string.Format("?oauth_token={0}&hd=default", oauth_token);
            System.Diagnostics.Process.Start(request2);//launch browser
            Console.WriteLine("Please input the pin code:");
            string pin = Console.ReadLine();
            //5)Upgrade to an Access Token:
            timestamp = GenerateTimeStamp();
            nonce = GenerateNonce();

            string request3 = OAuthGetAccessToken;
            StringBuilder sb3 = new StringBuilder();
            sb3.AppendFormat("oauth_consumer_key={0}", oauth_consumer_key);
            sb3.AppendFormat("&oauth_nonce={0}", nonce);
            sb3.Append("&oauth_signature_method=HMAC-SHA1");
            sb3.AppendFormat("&oauth_timestamp={0}", timestamp);
            sb3.AppendFormat("&oauth_token={0}", UrlEncode(oauth_token));
            sb3.AppendFormat("&oauth_verifier={0}", UrlEncode(pin));
            sb3.Append("&oauth_version=1.0");
            StringBuilder signatureBase3 = new StringBuilder("GET");
            signatureBase3.AppendFormat("&{0}", UrlEncode(OAuthGetAccessToken));
            signatureBase3.AppendFormat("&{0}", UrlEncode(sb3.ToString()));
            string sign3 = signatureBase3.ToString();
            Debug.WriteLine("Signature Base 3:" + sign3);
            string key3 = string.Format("{0}&{1}", UrlEncode(consumerSecret), UrlEncode(oauth_token_secret));
            HMACSHA1 hmacsha3 = new HMACSHA1();
            hmacsha3.Key = Encoding.UTF8.GetBytes(key3);
            byte[] dataBuffer3 = System.Text.Encoding.UTF8.GetBytes(sign3);
            byte[] hashBytes3 = hmacsha3.ComputeHash(dataBuffer3);
            string signature3 = Convert.ToBase64String(hashBytes3);
            string header3 = string.Format("OAuth oauth_version=\"1.0\", oauth_nonce=\"{0}\", oauth_timestamp=\"{1}\", oauth_consumer_key=\"{2}\", oauth_verifier=\"{3}\", oauth_token=\"{4}\", oauth_signature_method=\"HMAC-SHA1\", oauth_signature=\"{5}\"",
                 UrlEncode(nonce), UrlEncode(timestamp), UrlEncode(oauth_consumer_key), UrlEncode(pin), UrlEncode(oauth_token), UrlEncode(signature3));
            WebClient wc3 = new WebClient();
            wc3.Headers["Authorization"] = header3;
            string result3 = wc3.DownloadString(request3);
            Debug.Write(result3);
        }
        private static string GenerateTimeStamp()
        {
            TimeSpan ts = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0);
            return Convert.ToInt64(ts.TotalSeconds).ToString();
        }
        private static string GenerateNonce()
        {
            Random rnd = new Random(Environment.TickCount);
            return rnd.Next(123400, 9999999).ToString("X");
        }
        static string UrlEncode(string value)
        {
            if (value == null)
                return string.Empty;
            StringBuilder result = new StringBuilder();
            foreach (char symbol in value)
            {
                if ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_.~".IndexOf(symbol) != -1)
                {
                    result.Append(symbol);
                }
                else
                {
                    result.Append('%' + String.Format("{0:X2}", (int)symbol));
                }
            }
            return result.ToString();
        }
    }
}