private void btnDongBan_Click(object sender, EventArgs e)
{
btnDongBan.Visible = false;
btnMoBan.Visible = true;
btnDoiBan.Visible = false;
btnThanhToan.Enabled = false;
bllBanHang.DongBan(maBan);
bllBanHang.DeleteHoaDon(lblMaHoaDon.Text);
lblMaHoaDon.Text = string.Empty;
lblGioVao.Text = string.Empty;
fpnlBan.Controls.Clear();
LoadBan(cboViTri.SelectedValue.ToString());
lblTrangThai.Text = "Trống";
}