Help with accessing a globe variable on a form from other form problem, thank you

[lwong]

Hi,
I am sorry for poor programming skill.
I have a form. Inside, I declared a global variable loginflag, the initial value is false, when users login, the value becomes true.But when I accessed this variable in other form, it always shows false.

//declare 
public bool loginflag=false;

//update the loginflag to true
 private void btOK_Click(object sender, EventArgs e)
        {
            if (txtusers.Text == "")
               .......
                    
            else
             {
                      
            LoginFlag = true;
            cSingleton.Instance.Main.Show();
                 
             } 
        }

//create singleton for accessing between forms 
  public sealed class cSingleton
    {
        FMain _Main;
        FLogin _login;


        public cSingleton() 
        {
         _Main = new FMain();
      
        _login = new FLogin();

        }
        static readonly cSingleton _Instance  = new cSingleton();
       
        public static cSingleton Instance
        {
            get { return _Instance; }
        }
        public FMain Main
        {
            get { return _Main; }
        }
   

        public FLogin Login
        {
            get { return _login; }
        }
 
    }

//access loginflag from other form
              else
                {
                    while (OpenComPort())
                    {
                        if (dr == DialogResult.Cancel)
                        {
                            dr = DialogResult.Retry;
                            
                            return;
                     }
                       
                        MessageBox.Show(cSingleton.Instance.Login.LoginFlag .ToString());//always false. problem here
}

What causes this? Thank you.