private void BtnToevoegen_Click(object sender, EventArgs e)
        {
            UserDialog dia = new UserDialog(true);

            dia.ShowDialog();
            List <User> beheerUser = _gc.BeheerUser();

            listBox1.DataSource = beheerUser;
        }
        private void BtnWijzigen_Click(object sender, EventArgs e)
        {
            User       updateUser = (User)listBox1.SelectedItem;
            UserDialog dia        = new UserDialog(false, updateUser);

            dia.ShowDialog();
            List <User> beheerUser = _gc.BeheerUser();

            listBox1.DataSource = beheerUser;
        }