Why validating Peppol UBL e-invoices in .NET is harder than it looks

Why validating Peppol UBL e-invoices in .NET is harder than it looks When I started building InvoiceXML (a REST API for Peppol and EN 16931 e-invoice compliance), one of the first technical walls I hit was EN 16931 Schematron validation in .NET. I assumed it would be straightforward. It was not. This post is about that journey: what Peppol UBL validation actually requires under the hood, why .NET makes it particularly awkward, what the community workarounds look like, and why I eventually decide