Validate that user has logged in once at a time

Dear All,

I was given a task to validate that the user has logged in once at a time, i.e the user once logged in cannot login from same or other place until logs out.

Below is the code for the events on my Login Page:

//create object of global.asax class
Global obj = new Global();
 protected void Login_Click(object sender, EventArgs e)
        {  
            bool result=obj.GenerateSession(TextBox1.Text,true);
            if(result){
                Label1.Text = "Logged in Sucessfully";
            }
            else
            {
                Label1.Text = "You are already logged in";
            }
        }

        protected void Logout_Click(object sender, EventArgs e)
        {
            bool result=obj.DeleteValue(TextBox1.Text);
            if (result)
            {
                Label1.Text = "Logged Out Sucessfully";
            }
            else
            {
                Label1.Text = "You are already logged Out";
            }
        }


Below is the code of Global.asax file:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Optimization;
using System.Web.Routing;
using System.Web.Security;
using System.Net;

namespace Test
{
    public class Global : HttpApplication
    {
        static Dictionary<string, bool> SessionDict = new Dictionary<string, bool>();

        void Application_Start(object sender, EventArgs e)
        {
            // Code that runs on application startup
            BundleConfig.RegisterBundles(BundleTable.Bundles);
            AuthConfig.RegisterOpenAuth();
            RouteConfig.RegisterRoutes(RouteTable.Routes);
        }

        void Session_OnEnd(object sender, EventArgs e)
        {

        }

        public bool GenerateSession(string bname,bool val)
        {         
            bool _value = GetValue(bname);
            if (_value)
            {
                //not allowed to login as session already exists
                return false;
            }
            else
            {
                //loggingin for the first time
                SetValue(bname, val);
                return (GetValue(bname));
            }
        }

        public void SetValue(string key, bool value)
        {
            SessionDict.Add(key, value);
        }

        public bool DeleteValue(string _str)
        {
            bool value = false;
            if (SessionDict.ContainsKey(_str))
            {
                if (SessionDict[_str] != null)
                {
                    value = SessionDict[_str];
                    value=SessionDict.Remove(_str);
                    return value;
                }
                else
                {
                    return false;
                }
            }
            else
            {
                return false;
            }
        }


        //Get value from cokkie   
        public bool GetValue(string _str)
        {
            bool value = false;
            if (SessionDict.ContainsKey(_str))
            {
                if (SessionDict[_str] != null)
                {
                    value = SessionDict[_str];
                    return value;
                }
                else
                {
                    return false;
                }
            }
            else
            {
                return false;
            }
        }

        void Application_End(object sender, EventArgs e)
        {
            //  Code that runs on application shutdown

        }

        void Application_Error(object sender, EventArgs e)
        {
            // Code that runs when an unhandled error occurs

        }


    }
}