EN 16931 e-invoice validation for .NET

Read and validate EN 16931 electronic invoices in .NET. No Java, no Node, nothing leaves your server.

Since January 2025 every company in Germany must be able to receive an electronic invoice. If your software receives them, someone will send you a file and expect you to know whether it is valid.

Klarfakt does that in .NET, in your own process. It runs the publishers’ own compiled Schematron — the same artefacts the German reference validator executes — fetched from their pinned upstream releases and checked against a recorded SHA-256. Not a reimplementation of the rules, and not a copy vendored into a repository where it can drift.

100 of 100

Every CI run puts 143 invoices through both Klarfakt and KoSIT’s validationtool, using the same scenario configuration release, and diffs the rule ids each reports.

100 of 100 comparable files agree rule for rule. The other 43 are recorded with the reason they cannot be compared. A disagreement that is not written down fails the build.

That is the claim worth checking, and the script that proves it is in the repository.

What it covers

var validator = new InvoiceValidator();
var document = InvoiceDocument.Load("invoice.xml");   // or a Factur-X / ZUGFeRD PDF
var result = validator.Validate(document);

foreach (var error in result.Errors)
{
    Console.WriteLine($"{error.RuleId} at {error.Location}: {error.Message}");
}

Findings carry the rule id, severity, location, the failing XPath test, the business terms, the offending value where the rule pointed at one, and which artefact produced them — so a CEN core rule is distinguishable from a German CIUS rule.

The rule artefacts are not redistributed

The CEN rules are EUPL-1.2 and the KoSIT configurations are Apache-2.0. They stay with their publishers and are downloaded on request, over plain HTTPS from a pinned release, each file checked against the SHA-256 in the embedded manifest before it is written and again before first use.

Nothing else in Klarfakt touches the network. Validation is entirely local, and a Docker build can bake the artefacts into the image so the container runs with no network at all.

Writing

• Four hundred bytes — A 424-byte PDF that terminates a .NET process, why a catch cannot help, and the compression bomb next to it.

• Validating European e-invoices in .NET, without a JVM — Why XslCompiledTransform cannot run the EN 16931 artefacts, what SaxonCS-HE 13 changed, and four things that cost an afternoon.

All writing · Atom feed

Licence

Free unless your organisation turns over more than €1,000,000 a year. Free at any size inside OSI-licensed open source. Free always for evaluation, development, CI and internal testing.

Above that threshold and in production, a commercial licence is €690 a year — pricing. Every version becomes Apache 2.0 four years after it ships, so there is no lock-in if the project stops being maintained.

Source on GitHub · licensing@klarfakt.dev