Now part of the ZUGFeRD community:
https://github.com/zugferd


The ZUGFeRD library allows to create XML files as required by German electronic invoice initiative ZUGFeRD.

The library is meant to be as simple as possible, however it is not straight forward to use as the resulting XML file contains a complete invoice in XML format. Please take a look at the ZUGFeRD-Test project to find sample creation code. This code creates the same XML file as shipped with the ZUGFeRD information package.

The code used here is:

InvoiceDescriptor desc = InvoiceDescriptor.CreateInvoice("471102", new DateTime(2013, 6, 5), CurrencyCodes.EUR, "GE2020211-471102");
desc.Profile = Profile.Comfort;
desc.ReferenceOrderNo = "AB-312";
desc.AddNote("Rechnung gemäß Bestellung Nr. 2013-471331 vom 01.03.2013.");
desc.AddNote("Es bestehen Rabatt- und Bonusvereinbarungen.", "AAK");
desc.SetBuyer("Kunden Mitte AG", "69876", "Frankfurt", "Kundenstraße", "15", "DE", "88", "4000001987658");
desc.AddBuyerTaxRegistration("DE234567890", "VA");
desc.SetBuyerContact("Hans Muster");
desc.SetSeller("Lieferant GmbH", "80333", "München", "Lieferantenstraße", "20", "DE", "88", "4000001123452");
desc.AddSellerTaxRegistration("201/113/40209", "FC");
desc.AddSellerTaxRegistration("DE123456789", "VA");
desc.SetBuyerOrderReferenceDocument("2013-471331", new DateTime(2013, 03, 01));
desc.SetDeliveryNoteReferenceDocument("2013-51111", new DateTime(2013, 6, 3));
desc.ActualDeliveryDate = new DateTime(2013, 6, 3);
desc.SetTotals(202.76m, 5.80m, 14.73m, 193.83m, 21.31m, 215.14m, 50.0m, 165.14m);
desc.AddApplicableTradeTax(9.06m, 129.37m, 7m, "VAT", "S");
desc.AddApplicableTradeTax(12.25m, 64.46m, 19m, "VAT", "S");
desc.SetLogisticsServiceCharge(5.80m, "Versandkosten", "VAT", "S", 7m);
desc.setTradePaymentTerms("Zahlbar innerhalb 30 Tagen netto bis 04.07.2013, 3% Skonto innerhalb 10 Tagen bis 15.06.2013", new DateTime(2013, 07, 04));

desc.Save("output.xml");

A description of the library can be found here:
http://www.s2-industries.com/wordpress/2013/11/creating-zugferd-descriptors-with-c/

The solution is used in CKS.DMS and supported by CKSolution:
http://www.cksolution.de

You can find more information about ZUGFeRD here:
http://www.ferd-net.de/