login with google in .net

public static class GoogleAuthenticationApi
    {
        public static string GetAuthenticationToken(string userName, string password, out bool isUserValid)
        {
            string response = GoogleRequestHelper.GetPostResult("https://www.google.com/accounts/clientlogin", String.Format("accountType=HOSTED_OR_GOOGLE&Email={0}&Passwd={1}&service=lh2&souce=sourceappname", userName, password), out isUserValid);

            if (response.Contains("Auth="))
            {
                foreach (string line in response.Split(Environment.NewLine.ToCharArray()))
                {
                    if (line.StartsWith("Auth="))
                    {
                        response = line.Replace("Auth=", "");
                        break;
                    }
                }
            }
            else
            {
                response = "Error:" + response;
            }
            return response;
        }

        public static XmlDocument GetRoot(string feedurl, string auth)
        {
            XmlDocument doc = new XmlDocument();
            string result = "";
            HttpWebResponse response;
            WebRequest request = HttpWebRequest.Create(feedurl);

            if (!String.IsNullOrEmpty(auth))
                request.Headers.Add(HttpRequestHeader.Authorization, "GoogleLogin auth=" + auth);

            request.Method = "GET";
            response = (HttpWebResponse)request.GetResponse();

            if (response.StatusCode == HttpStatusCode.OK)
            {
                StreamReader sr = new StreamReader(response.GetResponseStream());
                result = sr.ReadToEnd();
                doc.LoadXml(result);
            }
            else
            {
                throw new Exception("Fault: Status code - " + response.StatusCode.ToString());
            }
            return doc;
        }
    }

    public class GoogleRequestHelper
    {
        public static string GetPostResult(string url, string strPost, out bool isUserValid)
        {
            string strResponse = "";
            try
            {
                UTF8Encoding objUTFEncode = new UTF8Encoding();
                byte[] arrRequest;
                Stream objStreamReq;
                StreamReader objStreamRes;
                HttpWebRequest objHttpRequest;
                HttpWebResponse objHttpResponse;
                Uri objUri = new Uri(url);

                objHttpRequest = (HttpWebRequest)HttpWebRequest.Create(objUri);
                objHttpRequest.KeepAlive = false;
                objHttpRequest.Method = "POST";

                objHttpRequest.ContentType = "application/x-www-form-urlencoded";
                arrRequest = objUTFEncode.GetBytes(strPost);
                objHttpRequest.ContentLength = arrRequest.Length;

                objStreamReq = objHttpRequest.GetRequestStream();
                objStreamReq.Write(arrRequest, 0, arrRequest.Length);
                objStreamReq.Close();

                //Get response
                objHttpResponse = (HttpWebResponse)objHttpRequest.GetResponse();
                objStreamRes = new StreamReader(objHttpResponse.GetResponseStream(), Encoding.ASCII);

                strResponse = objStreamRes.ReadToEnd();
                objStreamRes.Close();
                isUserValid = true;
            }
            catch (Exception ex)
            {
                strResponse = "Error:" + ex.Message;
                isUserValid = false;
            }
            return strResponse;
        }
    }