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class _Forms : ApplicationContext
    {
        public static string CurrentUser { get; set; }
        public static string CurrentPw { get; set; }

        public _Forms()
        {

            //Now the loop starts which control the Froms
            while (true)
            {
                LogIn loginForm = new LogIn();
                MainForm mainForm = new MainForm();
                DialogResult res = loginForm.ShowDialog();
                if (res != DialogResult.OK) break;
                mainForm.user = CurrentUser;
                mainForm.password = CurrentPw;
                res = mainForm.ShowDialog();
                if (res == DialogResult.Retry) continue;
                if (res == DialogResult.Cancel) break;
            }
            Application.Exit(); //hier sollte eigentlich schluss sein
        }

    }                    

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Application.Run();