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LogIn loginForm = new LogIn();
loginForm.Show();					

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static void Main()
{
  LogIn loginDlg = new LogIn();
  if (loginDlg.ShowDialog() == DialogResult.OK)
  {
     Application.Run(new MainForm());
  }
}					

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class _Forms : ApplicationContext
    {
        public static string CurrentUser { get; set; }
        public _Forms()
        {
            LogIn loginForm = new LogIn();
            MainForm mainForm = new MainForm();

            //Now the loop starts which control the Froms
            while (true)
            {
                DialogResult res = loginForm.ShowDialog();
                if (res != DialogResult.OK) break;
                mainForm.user = CurrentUser;
                res = mainForm.ShowDialog();
                if (res == DialogResult.Retry) continue;
                if (res == DialogResult.Cancel) break;
            }
            Application.Exit();
        }

    }