internal void WriteXml(XmlTextWriter writer)
        {
            writer.WriteStartElement(ElementName);
                writer.WriteElementString(CouponCodeElement, CouponCode);
                writer.WriteElementString(NameElement,Name);
                writer.WriteElementStringIfProvided(HostedDescriptionElement,HostedDescription);
                writer.WriteElementStringIfProvided(InvoiceDescriptionElement, InvoiceDescription);
                writer.WriteElementDateTimeIfProvided(RedeemByDateElement, RedeemByDate);
                writer.WriteElementString(SingleUseElement, SingleUse.ToString());
                writer.WriteElementIntIfProvided(AppliesForMonthsElement,AppliesForMonths);
                writer.WriteElementString(MaxRedemptionsElement, MaxRedemptions.ToString(CultureInfo.InvariantCulture));
                WriteDiscountElements(writer);
                if(!AppliesToAllPlans)
                {
                    writer.WriteElementString(AppliesToAllPlansElement, AppliesToAllPlans.ToString());
                    writer.WriteElementList(PlanCodeListElement, PlanCodes, WritePlanCodeItem);
                }

            writer.WriteEndElement();
        }
 protected void WriteCreateXml(XmlTextWriter writer, RecurlyAccount account, string couponCode, int? billingCycles, DateTime? firstRenewalDate)
 {
     writer.WriteStartElement(ElementName); // Start: subscription
         writer.WriteElementString(PlanCodeElement, PlanCode); //Required
         account.WriteXml(writer); //Required
         writer.WriteElementStringIfProvided(CouponCodeElement,couponCode);
         writer.WriteElementIntIfProvided(UnitAmountInCentsElement, UnitAmountInCents);
         writer.WriteElementString(CurrencyElement,Currency); //Required
         writer.WriteElementIntIfProvided(QuantityElement, Quantity > 1 ? Quantity : new int?());
         writer.WriteElementDateTimeIfProvided(TrialEndsAtElement, TrialEndsAt);
         writer.WriteElementDateTimeIfFuture(StartsAtElement,ActivatedAt);
         writer.WriteElementIntIfProvided(TotalBillingCyclesElement, billingCycles);
         writer.WriteElementDateTimeIfProvided(FirstRenewalDateElement, firstRenewalDate);
         writer.WriteElementListIfAny(AddonsElement, Addons, (w, a) => a.WriteXml(w));
         WriteManualInvoiceElement(writer);
     writer.WriteEndElement(); // End: subscription
 }