private void cbMaQua_SelectedIndexChanged(object sender, EventArgs e)
{
HangTrongDon hd = new HangTrongDon();
txtKho.Text = hd.LaySLKho(cbMaQua.SelectedValue.ToString()).ToString();
}
private void cbMaHang_SelectedIndexChanged(object sender, EventArgs e)
{
txtKho.Text = hd.LaySLKho(cbMaHang.SelectedValue.ToString()).ToString();
}