private void hàngHóaToolStripMenuItem1_Click(object sender, EventArgs e)
 {
     HangHoa frm = new HangHoa();
     frm.MdiParent = this;
     frm.Show();
 }
 private void buttonItem6_Click(object sender, EventArgs e)
 {
     HangHoa hh = new HangHoa();
        hh.MdiParent = this;
     hh.Show();
 }