private void buttonItem11_Click(object sender, EventArgs e)
 {
     HoaDonBan hdb = new HoaDonBan();
     hdb.MdiParent = this;
     hdb.Show();
        // this.Hide();
 }
 private void hóaĐơnBánToolStripMenuItem_Click(object sender, EventArgs e)
 {
     HoaDonBan frm = new HoaDonBan();
     frm.MdiParent = this;
     frm.Show();
 }