private void lậpHóaĐơnToolStripMenuItem_Click(object sender, EventArgs e)
{
LapHoaDon frm = new LapHoaDon();
frm.MdiParent = this;
frm.Show();
}
private void button2_Click(object sender, EventArgs e)
{
LapHoaDon frm = new LapHoaDon();
frm.ShowDialog();
}