Convert Markdown to PDF 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.
• A Markdown file. report.md next to the program, up to 1 MB. A sample is further down.

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

The program

The program reads report.md, posts it as markdown with bookmarks from its headings, waits for the result when the API answers 202, stops with the API’s message when something is wrong, and saves the PDF.

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

var body = new
{
    markdown = File.ReadAllText("report.md"),
    settings = new {
        pdf = new { outline = true },
    },
};

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("report.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, and report.pdf lies next to the program.

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.

A Markdown file with a title

The front matter gives the PDF its title and author; without one the first heading is the title. A line \newpage starts a new page, and a link to a heading works inside the PDF:

---
title: Release notes 2.4
author: Fennlor Studio GmbH
---

# Release notes 2.4

## What is new

- Faster previews, see [the numbers](#numbers)
- Footnotes in the export[^1]
- [x] Tested on every plan

## Numbers

| Step    | Before | After  |
| :------ | -----: | -----: |
| Preview | 840 ms | 310 ms |
| Export  | 2.1 s  | 1.4 s  |

\newpage

## Thanks

To everyone who reported a problem.

[^1]: From version 2.4 on.

What the settings control

• Look. settings.markdown.theme is document, which styles the text, in the brand kit’s fonts and colour where the workspace has one, or none; settings.markdown.css comes after it.
• Raw HTML. Shown as text by default. settings.markdown.html: "allow" writes it as it stands, for Markdown you wrote yourself, never for text from your users or a model.
• Line breaks. settings.markdown.breaks: true makes a single line break a line break, for text from a chat.
• Paper, header and footer. paper, margin, header.html and footer.html work as for HTML; elements with the classes pageNumber and totalPages show the page numbers.

Placeholders in the Markdown

Add "engine": "jinja2" (or liquid, handlebars) and a data object, and {{ … }} in the text is filled before it is read as Markdown, with all helpers. The Markdown guide has the rest.

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 Markdown to PDF API.

More C# (.NET) guides

• Generate a PDF invoice from JSON in C# (.NET)
• Create a ZUGFeRD / Factur-X e-invoice 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)