C# Class has login / logout complications.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Net;
using System.Web;
using System.Windows.Forms;


namespace Neopets.classes.communications
{
    public delegate void OnCriticalError(object source, CriticalError e);
    public class CriticalError : EventArgs
    {
        private string message;
        public CriticalError(string message)
        {
            this.message = message;
        }
        public string Message()
        {
            return this.message;
        }
    }

    class Authentication
    {
        public event OnCriticalError AuthCriticalError;

        CookieContainer cookies = new CookieContainer();

        private bool is_Loggedin()
        {
            return RequestPage("http://www.neopets.com/index.phtml", "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0", "").Contains("Logout");
        }

        private void AquireCookies(string url)
        {
            HttpWebRequest request = null;
            HttpWebResponse response = null;

            request = (HttpWebRequest)WebRequest.Create(new Uri(url));
            request.Method = "GET";
            request.CookieContainer = cookies;

            try
            {
                response = (HttpWebResponse)request.GetResponse();
            }
            catch (WebException e)
            {
               AuthCriticalError(this, new CriticalError(e.Message));
            }
        }

        private string RequestPage(string uri, string UserAgent, string Refer)
        {

            HttpWebRequest request = null;
            HttpWebResponse response = null;

            // used to build entire input
            StringBuilder sb = new StringBuilder();

            // used on each read operation
            byte[] buf = new byte[8192];

            // prepare the web page we will be asking for
            request = (HttpWebRequest)WebRequest.Create(uri);
            request.UserAgent = UserAgent;
            request.Referer = Refer;
            request.CookieContainer = cookies;

            // execute the request

            try
            {
                response = (HttpWebResponse)request.GetResponse();

                // we will read data via the response stream
                Stream resStream = response.GetResponseStream();

                string tempString = null;
                int count = 0;

                do
                {
                    // fill the buffer with data
                    count = resStream.Read(buf, 0, buf.Length);

                    // make sure we read some data
                    if (count != 0)
                    {
                        // translate from bytes to ASCII text
                        tempString = Encoding.ASCII.GetString(buf, 0, count);

                        // continue building the string
                        sb.Append(tempString);
                    }
                }
                while (count > 0); // any more data to read?

                // print out page source
                return (sb.ToString());
            }
            catch (WebException e)
            {
                AuthCriticalError(this, new CriticalError(e.Message));
                return e.Message;
            }
        }

        private string PostForm(StringBuilder postData, string uri, string UserAgent, string Referer, string ContentType = "application/x-www-form-urlencoded", bool AllowAutoRedirect = true, bool KeepAlive = true)
        {

            HttpWebRequest request = null;
            HttpWebResponse response = null;
            AquireCookies(uri);

            request = (HttpWebRequest)WebRequest.Create(new Uri(uri));
            request.ContentType = ContentType;
            request.UserAgent = UserAgent;
            request.Referer = Referer;
            request.AllowAutoRedirect = AllowAutoRedirect;
            request.KeepAlive = KeepAlive;
            request.CookieContainer = cookies;
            request.Method = "POST";

            //write the POST data to the stream
            try
            {
                using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))
                {
                    writer.Write(postData.ToString());
                }

                response = (HttpWebResponse)request.GetResponse();

                using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                {
                    return reader.ReadToEnd();
                }
            }
            catch(WebException e)
            {
                AuthCriticalError(this, new CriticalError(e.Message));
                return e.Message;
            }

        }

        public int Login(string username, string password)
        {
            if (is_Loggedin() == false)
            {
                StringBuilder PostData = new StringBuilder();
                PostData.Append("destination=%252Findex.phtml");
                PostData.Append("&username=" + HttpUtility.UrlEncode(username));
                PostData.Append("&password=" + HttpUtility.UrlEncode(password));

                string returnData = PostForm(PostData, "http://www.neopets.com/login.phtml", "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0", "http://www.neopets.com", "application/x-www-form-urlencoded", true, true);

                if (returnData.Contains("Incorrect username in cookie."))
                {
                    return 1;
                }

                if (returnData.Contains("Sorry, we did not find an account with that username") || returnData.Contains("No username found! Please go back and re-enter your username.") || returnData.Contains("Sorry, but you have invalid characters in your username."))
                {
                    return 2;
                }

                if (returnData.Contains("Invalid Password. Please enter the correct password to continue."))
                {
                    return 3;
                }

                if (returnData.Contains("Logout"))
                {
                    return 0;
                }

                return 4;
            }
            else
            {
                return 0;
            }
        }

        public void Logout()
        {
            if (is_Loggedin() == true)
            {
                RequestPage("http://www.neopets.com/logout.phtml", "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0", "");
            }
            if (is_Loggedin() == false)
            {
                MessageBox.Show("You Have Logged Out");
            }
        }
    }
}