takes a refreshtoken and exchanges it for an accessotken.

getAcccessToken

   class AccessTokenResponse
    {
        public string access_token { get; set; }
        public string token_type { get; set; }
        public string expires_in { get; set; }
    }
  
  /// <summary>
        /// takes a refreshtoken and exchanges it for an accessotken.
        /// </summary>
        /// <param name="refreshToken"></param>
        /// <returns></returns>
        public static string getAcccessToken(string refreshToken)
        {
            string url = "https://accounts.google.com/o/oauth2/token";
            string postData = string.Format("client_id={0}&client_secret={1}&refresh_token={2}&grant_type=refresh_token", Properties.Resources.clientId, Properties.Resources.secret, refreshToken);


            // Create a request using a URL that can receive a post. 
            WebRequest request = WebRequest.Create(url);
            // Set the Method property of the request to POST.

            request.Method = "POST";

            byte[] byteArray = Encoding.UTF8.GetBytes(postData);
            // Set the ContentType property of the WebRequest.
            request.ContentType = "application/x-www-form-urlencoded";
            // Set the ContentLength property of the WebRequest.
            request.ContentLength = byteArray.Length;
            // Get the request stream.
            Stream dataStream = request.GetRequestStream();
            // Write the data to the request stream.
            dataStream.Write(byteArray, 0, byteArray.Length);
            // Close the Stream object.
            dataStream.Close();
            // Get the response.
            WebResponse response = request.GetResponse();
            // Display the status.
            Console.WriteLine(((HttpWebResponse)response).StatusDescription);
            // Get the stream containing content returned by the server.
            dataStream = response.GetResponseStream();
            // Open the stream using a StreamReader for easy access.
            StreamReader reader = new StreamReader(dataStream);
            // Read the content.
            string responseFromServer = reader.ReadToEnd();

            AccessTokenResponse tmp = JsonConvert.DeserializeObject<AccessTokenResponse>(responseFromServer);

            // Display the content.
            Console.WriteLine(responseFromServer);
            // Clean up the streams.
            reader.Close();
            dataStream.Close();
            response.Close();
            return tmp.access_token;

        }