private void bánHàngToolStripMenuItem_Click(object sender, EventArgs e)
 {
     DatHang frm = new DatHang();
     frm.MdiParent = this;
     frm.Show();
 }
        private void buttonItem9_Click(object sender, EventArgs e)
        {
            DatHang dh = new DatHang();

            dh.MdiParent = this;
            dh.Show();
               // this.Hide();
        }