private void sairToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.Hide();

            SegurancaForm f = Application.OpenForms["SegurancaForm"] as SegurancaForm;

            f.Show();
        }
        private void PrincipalForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            DialogResult dg;

            using (DialogCenteringService centeringService = new DialogCenteringService(this)) // center message box
            {
                dg = MessageBox.Show(this,
                                     "Tem certeza que deseja sair?",
                                     "Encerrar programa",
                                     MessageBoxButtons.YesNo, MessageBoxIcon.Question);
            }

            if (dg == DialogResult.No)
            {
                e.Cancel = true;
            }
            else
            {
                SegurancaForm f = Application.OpenForms["SegurancaForm"] as SegurancaForm;

                f.Close();
            }
        }