private void cboChoose_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (cboChoose.SelectedIndex == -1)
            {
                return;
            }

            px.Search = cboChoose.Text;
            cboSearch.Items.Clear();
            List <PhieuXuatHang> pxlist = pxbll.LocDanhSachPhieuXuatHang(px);

            if (cboChoose.Text == "Mã Phiếu Xuất")
            {
                cboSearch.Text = "";
                foreach (PhieuXuatHang pxs in pxlist)
                {
                    cboSearch.Items.Add(pxs.MaPx);
                }
            }
            else if (cboChoose.Text == "Mã Nhân Viên")
            {
                cboSearch.Text = "";
                foreach (PhieuXuatHang pxs in pxlist)
                {
                    cboSearch.Items.Add(pxs.MaNv);
                }
            }
            else if (cboChoose.Text == "Mã Sản Phẩm")
            {
                cboSearch.Text = "";
                foreach (PhieuXuatHang pxs in pxlist)
                {
                    cboSearch.Items.Add(pxs.MaSp);
                }
            }
            else if (cboChoose.Text == "Mã Khách Hàng")
            {
                cboSearch.Text = "";
                foreach (PhieuXuatHang pxs in pxlist)
                {
                    cboSearch.Items.Add(pxs.MaKh);
                }
            }
            else
            {
                return;
            }
        }