csharp

CII for C# and .NET: InvoiceXML REST API Examples

C# and .NET code samples for creating, validating, and parsing CII electronic invoices using the invoicexml.com API. Requires .NET 6 or later (.NET 8 recommended), and ready to drop into console apps, ASP.NET Core Web APIs, Blazor, MAUI, Azure Functions, or AWS Lambda.

Get your API key

Every example in this folder calls the InvoiceXML REST API. Sign up and generate a key here:

→ https://www.invoicexml.com/account/authentication

Pass it as a Bearer token on every request:

Authorization: Bearer YOUR_API_KEY

Important: pass the raw key only, without the Bearer prefix. If your account page shows the full header value (e.g. Bearer ixml_a1b2c3...), copy only the part after Bearer . Flurl's WithOAuthBearerToken(apiKey) adds the prefix itself.

Requirements

• .NET 6.0 or later (recommended: .NET 8) with ImplicitUsings enabled (the default in new SDK-style projects)
• Runs on Windows, Linux, macOS, Docker, Azure, and AWS Lambda
• On .NET Framework 4.7.2+ the API calls work too (Flurl.Http targets .NET Standard 2.0), but the files as written need small changes: add the using System; using System.IO; using System.Threading.Tasks; directives and replace File.WriteAllBytesAsync / File.WriteAllTextAsync (not available on .NET Framework) with the synchronous File.WriteAllBytes / File.WriteAllText
• One NuGet package: Flurl.Http for clean multipart uploads and bearer auth

dotnet add package Flurl.Http

If you prefer plain HttpClient over Flurl, every example translates directly. Flurl just makes the multipart and authentication wiring a one-liner.

Files in this folder

File	Operation	API endpoint
Create.cs	Build a CII D16B XML invoice	POST /v1/create/cii
Validate.cs	Validate a CII file against the D16B XSD and EN 16931 rules	POST /v1/validate/cii
ExtractJson.cs	Parse a CII XML into JSON	POST /v1/extract/json
AiConvert.cs	(Experimental) Convert a plain PDF to CII with AI	POST /v1/transform/to/cii
Render.cs	Render CII XML into a human-readable PDF	POST /v1/render/cii/to/pdf

Note on the snippets below: they are excerpts from those files and assume an apiKey variable is already defined and that the code runs inside an async method. The full files show the complete, compilable versions.

---

Create a CII invoice in C#

using Flurl.Http;

var payload = new
{
    invoice = new
    {
        invoiceNumber = "CII-2026-001",
        issueDate     = "2026-05-18",
        currency      = "EUR",
        seller = new
        {
            name              = "Acme GmbH",
            vatIdentifier     = "DE123456789",
            legalRegistration = new { identifier = "HRB 12345" },
            postalAddress     = new { line1 = "Hauptstraße 12", city = "Berlin", postCode = "10115", country = "DE" }
        },
        buyer = new
        {
            name          = "Globex SAS",
            postalAddress = new { line1 = "15 rue de Rivoli", city = "Paris", postCode = "75001", country = "FR" }
        },
        paymentDetails = new { paymentAccountIdentifier = "DE89370400440532013000" },
        lines = new[]
        {
            new {
                quantity       = 10,
                priceDetails   = new { netPrice = 150.00m },
                vatInformation = new { rate = 19.00m },
                item           = new { name = "Senior consulting" }
            }
        }
    }
};

var xml = await "https://api.invoicexml.com/v1/create/cii"
    .WithOAuthBearerToken(apiKey)
    .PostJsonAsync(payload)
    .ReceiveString();

await File.WriteAllTextAsync("invoice-cii.xml", xml);

Totals and the VAT breakdown are calculated from the line items when omitted, and the document declares urn:cen.eu:en16931:2017 in BT-24.

The response is the CII D16B XML document, validated against the EN 16931 rules before delivery.

Full example: Create.cs | API reference

---

Validate a CII file in C#

using Flurl.Http;

var report = await "https://api.invoicexml.com/v1/validate/cii"
    .WithOAuthBearerToken(apiKey)
    .PostMultipartAsync(mp => mp
        .AddFile("file", "invoice.xml", "application/xml"))
    .ReceiveString();

Console.WriteLine(report);

Returns a JSON validation report listing any rule failures (EN 16931 BR-* and BR-CO-* rules).

Full example: Validate.cs | API reference

---

Extract CII data as JSON in C#

A common need: get CII invoice data into a JSON-friendly format for REST APIs, ERPs, or downstream pipelines.

using Flurl.Http;

var json = await "https://api.invoicexml.com/v1/extract/json"
    .WithOAuthBearerToken(apiKey)
    .PostMultipartAsync(mp => mp
        .AddFile("file", "invoice.xml", "application/xml"))
    .ReceiveString();

await File.WriteAllTextAsync("invoice.json", json);

Full example: ExtractJson.cs | API reference | Sample response

---

(Experimental) Convert a plain PDF to CII with AI

Experimental feature. Human verification required before any production use.

Real-world PDF invoices are often messy: scanned at low quality, irregularly formatted, multi-page, multilingual, or missing fields that EN 16931 requires. AI extraction can make subtle mistakes that automated validators may not catch: wrong tax category codes, transposed amounts, missing seller VAT identifiers, incorrect currency formatting.

Always review the output before sending it to a customer or tax authority. See the AI conversion notes in the main README.

The endpoint takes the PDF alone; no extra parameters are needed.

using Flurl.Http;

var xml = await "https://api.invoicexml.com/v1/transform/to/cii"
    .WithOAuthBearerToken(apiKey)
    .PostMultipartAsync(mp => mp
        .AddFile("file", "plain-invoice.pdf", "application/pdf"))
    .ReceiveString();

await File.WriteAllTextAsync("converted-cii.xml", xml);

Full example: AiConvert.cs | API reference

---

Render CII as a readable PDF in C#

CII is a machine format with no visual layer, so nobody can read it without tooling. This endpoint renders the XML into a formatted PDF preview, generated fresh from the structured data rather than extracted from any existing PDF layer, which makes it a quick way to eyeball what your XML actually says. The PDF is for reading only; the XML file remains the authoritative invoice for compliance and tax purposes.

using Flurl.Http;

var pdfBytes = await "https://api.invoicexml.com/v1/render/cii/to/pdf"
    .WithOAuthBearerToken(apiKey)
    .PostMultipartAsync(mp => mp
        .AddFile("file", "invoice.xml", "application/xml")
        .AddString("language", "en")   // en, de, or fr
    )
    .ReceiveBytes();

await File.WriteAllBytesAsync("invoice-preview.pdf", pdfBytes);

Full example: Render.cs | API reference

---

Framework integration

ASP.NET Core / Web API

Return a CII invoice from a controller action by proxying the invoicexml.com API:

[HttpGet("invoices/{id}/cii")]
public async Task<IActionResult> GetCII(string id)
{
    var xml = await CreateCII.RunAsync(/* invoice data from your DB */);
    return File(System.Text.Encoding.UTF8.GetBytes(xml), "application/xml", $"invoice-{id}.xml");
}

Console / Worker Service

Each example exposes a static RunAsync method. From Program.cs:

await CreateCII.RunAsync();
await ValidateCII.RunAsync("invoice.xml");
await ExtractJson.RunAsync("invoice.xml");

---

Common issues

• 401 Unauthorized: API key missing or invalid. Generate one at invoicexml.com/account/authentication and confirm you are sending Authorization: Bearer YOUR_API_KEY. A frequent cause: passing the whole Bearer xxx value as the key, which sends Bearer Bearer xxx. Pass the raw key only.
• 400 Bad Request on Create: a required field is missing or malformed. Frequent causes: IssueDate not in ISO format (YYYY-MM-DD), Currency not in ISO 4217 (EUR, USD), country codes not in ISO 3166-1 alpha-2 (DE, FR).
• Schematron BR-CO- failures on Validate*: line totals do not match the header total, or tax category and tax percentage are inconsistent. Recompute totals or leave empty for auto-calculation when posting.

---

Resources

• Get an API key
• Create CII API reference
• Validate CII API reference
• Extract JSON API reference
• Extract XML API reference
• Transform to CII API reference
• Flurl.Http documentation
• Main repository README