test.cs

public byte[] ZugferdXML {
  get {
    var invoice = InvoiceDescriptor.CreateInvoice(this.Id.ToString(),
        this.OrderDate, CurrencyCodes.USD);

    invoice.SetBuyer(this.Buyer.Name,
      this.Buyer.PostalCode,
      this.Buyer.City,
      this.Buyer.Address,
      this.Buyer.CountryCodes,
      this.Buyer.Id.ToString());

    invoice.SetSeller(this.Seller.Name,
      this.Seller.PostalCode,
      this.Seller.City,
      this.Seller.Address,
      this.Seller.CountryCodes,
      this.Seller.Id.ToString());

    invoice.SetSellerContact($"{this.Seller.BuyerContact.FirstName} {this.Seller.BuyerContact.LastName}");

    foreach (LineItem lineItem in this.LineItems) {
      invoice.AddTradeLineItem(lineItem.Name, lineItem.Description, lineItem.QuantityCode, lineItem.Quantity, lineItem.GrossPrice, lineItem.GrossPrice, lineItem.Quantity);
    }

    invoice.LineTotalAmount = LineTotalAmount;
    invoice.ChargeTotalAmount = invoice.LineTotalAmount;
    invoice.AllowanceTotalAmount = Allowance;
    invoice.TaxBasisAmount = invoice.ChargeTotalAmount;
    invoice.TaxTotalAmount = TaxTotalAmount;
    invoice.GrandTotalAmount = GrandTotalAmount;

    MemoryStream ms = new MemoryStream();

    invoice.Save(ms, ZUGFeRDVersion.Version1, Profile.Basic);

    return ms.ToArray();
  }
}