public PurchaseOrder Build()
        {
            var purchaseOrder = new PurchaseOrder(new OrderNo(_orderNo), _supplier);

            purchaseOrder.AddLineItems(_lineItems);

            if (_shipment != null)
            {
                purchaseOrder.SetPrivatePropertyValue(nameof(purchaseOrder.Shipment), _shipment);
            }

            return(purchaseOrder);
        }