Create a ZUGFeRD / Factur-X e-invoice in C# (.NET)

What you need

• An API key. Create one in the app under API keys and export it as FORMFEED_API_KEY. A test key (ff_test_…) is right for trying things out. See the quick start.
• C# (.NET). You need the .NET 8 SDK; the program uses System.Net.Http.Json and System.Text.Json, which ship with it.
• The template. e-rechnung from the e-invoice example: copy it into your workspace from the example’s page, or push the folder with the CLI (formfeed templates push --publish). Any invoice template works the same way once its data has an _invoice block.
• The data. The example’s data.json next to the program: one _invoice block with seller, buyer, lines, the VAT breakdown and the totals, in the terms of the European standard EN 16931.
• A plan with e-invoices. Live e-invoices need the Starter plan or above. With a test key (ff_test_…) every plan makes test e-invoices with a test mark on the page, the Free plan up to 20 a day.

dotnet new console -o render && cd render   # then replace Program.cs

The program

The program reads data.json, posts it with the template’s name and post.einvoice, waits for the result when the API answers 202, stops with the API’s message when something is wrong, and saves the e-invoice.

using System.Net.Http.Headers;
using System.Net.Http.Json;
using System.Text.Json;

var data = JsonDocument.Parse(File.ReadAllText("data.json")).RootElement;
var body = new
{
    template = "e-rechnung",
    data,
    post = new {
        einvoice = new { profile = "en16931" },
    },
};

using var http = new HttpClient();

// one call to the API: the key goes on each request, never on the client, so no other address sees it
async Task<HttpResponseMessage> Send(HttpMethod method, string url, object? content = null)
{
    using var request = new HttpRequestMessage(method, url) { Content = content is null ? null : JsonContent.Create(content) };
    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("FORMFEED_API_KEY"));
    return await http.SendAsync(request);
}

var response = await Send(HttpMethod.Post, "https://api-eu.formfeed.dev/v1/renders", body);
var render = await response.Content.ReadFromJsonAsync<JsonElement>();

// not done within 60 seconds: the answer is 202 and the render goes on in the background;
// its result answers the same way until the render has finished
while ((int)response.StatusCode == 202)
{
    await Task.Delay(2000);
    response = await Send(HttpMethod.Get, $"https://api-eu.formfeed.dev/v1/renders/{render.GetProperty("id")}/result");
    render = await response.Content.ReadFromJsonAsync<JsonElement>();
}

// every error is a problem document (RFC 9457) with a title and a detail
if (!response.IsSuccessStatusCode)
{
    Console.Error.WriteLine($"{render.GetProperty("title")}: {(render.TryGetProperty("detail", out var detail) ? detail : "")}");
    return 1;
}

await File.WriteAllBytesAsync("e-invoice.pdf", await http.GetByteArrayAsync(render.GetProperty("download_url").GetString()));
Console.WriteLine($"{render.GetProperty("id")} {render.GetProperty("page_count")} page(s), {render.GetProperty("units")} unit(s)");
return 0;

Run it

It prints the render’s id, the page count and the units it cost (on a live key one more than the same PDF without the XML), and e-invoice.pdf lies next to the program. A PDF viewer shows the invoice, and its attachments show the factur-x.xml inside. With a test key the render is not metered, and every test e-invoice carries a test mark, on every plan.

export FORMFEED_API_KEY=ff_test_...
dotnet run

What is particular about C# (.NET)

• Anonymous objects. The API’s field names are snake_case and are written as they are (webhook_url = …), so no naming policy has to be configured.
• Data without classes. JsonDocument.Parse(…).RootElement embeds the sample data untouched; in your application this is your own record or DTO.
• Key on the request, not the client. The Authorization header is set on each request to the API, never on the HttpClient: the download goes to a signed CDN URL and must not carry your API key.

What makes the PDF an e-invoice

• post.einvoice. turns the PDF into a PDF/A-3 that carries the invoice as factur-x.xml. The example template declares the same in its settings.json, so for this template the line is optional; for your own invoice template it is the one switch.
• The _invoice block. Seller, buyer, lines, VAT breakdown and totals, named as in the standard; units and VAT categories are codes (HUR, S). The template prints the page from the same block, so the page and the XML cannot say different things.
• The check. Before the file is delivered, the XML is held to the rules of EN 16931 and the PDF to PDF/A-3b. The report is in the render’s einvoice.validation, and a value the page does not show is listed in einvoice.display.missing.
• ZUGFeRD or Factur-X. One specification under two names, and the same file. flavour decides only which name and release the render reports: ZUGFeRD 2.5 or Factur-X 1.09.

When the data is wrong

A block that breaks a rule fails the request with 422 einvoice_data_invalid before anything is rendered or counted. The problem lists every finding with its place in the data and the rule it breaks (BR-CO-15, …), and the program prints its title and detail. A file that fails the check is never delivered, and never replaced by a plain PDF.

What stays with you

Formfeed creates and checks the file. Sending it to your customer, archiving it and the VAT treatment of the invoice stay with you. When an e-invoice is required, with the official sources, is on the e-invoicing page; the fields and the rules are in the e-invoice docs.

When the API says no

• One shape for every error. Errors are problem documents (RFC 9457) with type, title, status, detail, a stable code and the request_id to quote to support; the type URL leads to a page about that error. The list is in the error reference.
• The usual ones. 401 unauthorized for a missing or revoked key, 404 template_not_found for a wrong template name, 422 template_runtime_error when the template fails on the data, 429 rate_limited above the plan’s requests per second, 402 quota_exceeded when the Free plan’s units are used up (a paid plan bills extra units up to its hard cap).
• Retry safely. Send an Idempotency-Key header with the POST, and repeat the request with the same key when you do not know whether the first attempt arrived: for 24 hours the key returns the first answer instead of rendering and billing again. The answer is stored once the first attempt has been answered, so do not retry while it may still run; and problems below 500 are stored too, so a corrected request needs a new key.

The job around this program

This guide is one program. What the job needs around it, from the template to the limits and the price, is on the page Invoice PDF API.

More C# (.NET) guides

• Generate a PDF invoice from JSON in C# (.NET)
• Convert HTML to PDF in C# (.NET)
• Generate Open Graph images in C# (.NET)
• Generate PDFs from a CSV in C# (.NET)
• Merge PDFs and add a password in C# (.NET)
• Async PDF rendering with a verified webhook in C# (.NET)

FAQ

Which profile should I choose?

en16931, the default, carries every field of the European standard; basic is the smallest profile that is an invoice on its own. Any other profile is answered with einvoice_profile_unsupported.

Does Formfeed create XRechnung?

No. Formfeed writes ZUGFeRD / Factur-X: the XML inside a PDF/A-3. XRechnung and UBL are not available.

What does an e-invoice cost?

One unit on top of its render, so a one-page e-invoice costs two units. Live e-invoices need the Starter plan or above; on the Free plan, test renders make up to 20 a day. See pricing.

Can I have the XML as a file of its own?

xml: "both" in post.einvoice stores the XML beside the PDF as well, and the render links it as einvoice.xml_url. Inside the PDF it is there either way.

ZUGFeRD e-invoice in other languages

• Node.js (TypeScript)
• Python
• PHP
• Go
• Java
• Ruby
• curl