private void btDangNhap_Click(object sender, EventArgs e)
        {
            formDangNhap loginForm = new formDangNhap();

            loginForm.Show();
            Visible = false;
        }
 private void formDangNhap_FormClosed(object sender, FormClosedEventArgs e)
 {
     if (MessageBox.Show("Do you really want to exit ?", "Notification", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes)
     {
         Application.Exit();
     }
     else
     {
         formDangNhap fDN = new formDangNhap();
         fDN.Show();
     }
 }