private void btnHD_Click(object sender, EventArgs e)
 {
     frmHoaDon HD = new frmHoaDon();
     HD.ShowDialog();
 }
 private void LapHoaDonThanhToanToolStripMenuItem_Click(object sender, EventArgs e)
 {
     frmHoaDon Child = new frmHoaDon();
     Child.MdiParent = this;
     Child.Show();
 }