how to extend the DP class methods in D365

[ExtensionOf(ClassStr(PurchPurchaseOrderDP))]

final class PurchPurchaseOrderDP_Extension

{

   

    private void new()

    {

    }

 Protected PurchPurchaseOrderTmp initializeOrderLine(

        PurchPurchaseOrderHeader                    _purchaseOrderHeader,

        boolean                                     _highlightUpdated,

        PurchPurchaseOrderDPOrderLineQuerySelection _orderLineSelection)

    {

        PurchPurchaseOrderTmp purchPurchaseOrderTmp = next initializeOrderLine(_purchaseOrderHeader,

                                                                               _highlightUpdated,

                                                                               _orderLineSelection);

        if (purchPurchaseOrderTmp.PurchOrderTransOrTaxTrans)

        {

            //purchPurchaseOrderTmp.TRI_LineIsTaxed = this.TRILineIsTaxed(purchLine.RecId);

            purchPurchaseOrderTmp.TRI_LineIsTaxed = this.TRILineIsTaxed(PurchLine::findInventTransId(_orderLineSelection.parmPurchLineAllVersions().InventTransId).RecId, purchPurchaseOrderTmp);

        }

        //purchPurchaseOrderTmp.TRI_AGLRevision   = PurchLine::findRecId(purchline.RecId).TRI_AGLRevision;

        purchPurchaseOrderTmp.TRI_AGLRevision   = PurchLine::findRecId(PurchLine::findInventTransId(_orderLineSelection.parmPurchLineAllVersions().InventTransId).RecId).TRI_AGLRevision;

        return purchPurchaseOrderTmp;

    }

    /// <summary>

    ///  Set boolean if line is taxed

    /// </summary>

    /// <param name = "_purchLineRecId">RecId for purchLine</param>

    /// <returns>Returns true if line is taxed</returns>

    protected boolean TRILineIsTaxed(RefRecId _purchLineRecId, PurchPurchaseOrderTmp _purchPurchaseOrderTmp)

    {

        boolean                 lineIsTaxed;

        PurchTotals             tri_PurchTotals;

        TaxPurch                taxPurch;

        AmountCur               taxAmountCurTotal;

        AmountCur               taxRegulationTotalCur;

        ;

        tri_PurchTotals = PurchTotals::newPurchTable(VendPurchOrderJour::findRecId(_purchPurchaseOrderTmp.JournalRecId).purchTable());

        tri_PurchTotals.calc();

        taxPurch = tri_PurchTotals.tax();

        if (tri_PurchTotals && _purchLineRecId)

        {

            [taxAmountCurTotal, taxRegulationTotalCur] = taxPurch.showTaxesSourceSingleLine(tableNum(PurchLine), _purchLineRecId, true);

            lineIsTaxed = taxAmountCurTotal != 0.00;

        }

        return lineIsTaxed;

    }

}