VatEvidence.Vies 1.0.0

eu-vat-evidence

Open-source .NET library for EU VAT number validation and compliance evidence.

Built for developers who need reliable, auditable VAT handling in European B2B applications — without pulling in heavy dependencies or cloud services for basic format checks.

Why this exists

EU VAT compliance is a solved problem in theory and a surprisingly fragmented one in practice. Most libraries either:

• validate format only, with no path to active-status verification
• call VIES directly without local pre-validation, wasting network round-trips on obviously invalid numbers
• bundle everything into one package, forcing you to take HTTP clients when you only need a regex

This project takes a different approach: two focused NuGet packages with zero external dependencies, designed to be composed rather than consumed whole.

Packages

VatEvidence.Core

Local EU VAT number validation. No network calls, no external dependencies.

var result = VatNumberValidator.Validate("HR12345678901");
// VatValidationResult { IsValid, CountryCode, NormalizedVat, ErrorReason }

Covers all 27 EU member states with format validation and checksum verification where applicable (ISO 7064 MOD-11-10, MOD-97, MOD-11, Luhn). Results are deterministic, allocation-efficient, and safe to call from hot paths.

VatEvidence.Vies

EU VIES active-status verification. Calls the EU Commission VIES REST API to confirm whether a VAT number belongs to an active, registered business.

var vies = await viesClient.CheckAsync("HR", "12345678901", cancellationToken);
// ViesResult { IsActive, Name, Address, RequestDate, ErrorReason }

Intentionally decoupled from VatEvidence.Core — take one, the other, or both.

Validation flow

Raw input: "HR12345678901"
         │
         ▼
VatEvidence.Core                 ← fast, local, no network
VatNumberValidator.Validate()
         │
    ┌────┴────┐
  INVALID   VALID
    │          │
  return     VatEvidence.Vies    ← only if format passes
  error      ViesClient.CheckAsync()
                  │
             IsActive, Name, Address

This two-step design means invalid numbers never reach the network, and VIES quota is not wasted on typos or test data.

Stack

• .NET 10
• ASP.NET Core Minimal API — reference implementation
• xUnit — per-country unit tests

Project structure

eu-vat-evidence/
│
├── src/
│   ├── VatEvidence.Core/      # NuGet — validator, country classification
│   └── VatEvidence.Vies/      # NuGet — VIES HTTP client
│
├── VatEvidence.Web/           # Minimal API reference implementation
└── VatEvidence.Test.Unit/     # Per-country validator tests

Country coverage

All 27 EU member states, validated against official EU VAT format specifications:

Getting started

dotnet add package VatEvidence.Core
dotnet add package VatEvidence.Vies   # optional

// Step 1 — local validation, always
var result = VatNumberValidator.Validate(rawInput);

if (!result.IsValid)
    return BadRequest(result.ErrorReason);

// Step 2 — VIES check, only when needed
var vies = await _viesClient.CheckAsync(
    result.CountryCode!,
    result.NormalizedVat![2..],
    cancellationToken);

Register VIES client in DI:

builder.Services.AddHttpClient<IViesClient, ViesClient>();

Evidence records

Beyond validation, the project maintains an append-only SHA-256 hash-chain of VAT check events — designed for audit trails in invoicing and compliance systems where you need to prove a VAT number was valid at the time of a transaction.

License

MIT — see LICENSE.