private void cboMaDh_SelectedIndexChanged(object sender, EventArgs e)
{
if (cboMaDh.SelectedIndex == -1)
{
return;
}
px.MaDh = int.Parse(cboMaDh.Text);
cboSoLuongXuat.DataSource = pxbll.ShowSlxTheoMaDh(px);
cboMaSp.DataSource = pxbll.ShowMaSpTheoMaDh(px);
}