Generate PDFs from a CSV 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. certificate from the certificate example, whose fields are exactly the columns of the CSV.
• The CSV. attendees.csv next to the program, with a header row. It is shown below.
• Enough units. Batches are part of every plan, and each item counts as at least one unit. On the Free plan a live batch is refused when the month has fewer units left than the batch has items; a test key is not counted. The limits page has the units per plan.

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

The program

The program reads the CSV, turns every row into an item, queues them as one job with zip: true, waits until the job is done and saves the archive.

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

using var http = new HttpClient { BaseAddress = new Uri("https://api-eu.formfeed.dev/v1/") };

// one call to the API: the key goes on each request, never on the client, so the zip's address does not see it
async Task<JsonElement> Api(HttpMethod method, string path, object? body)
{
    using var request = new HttpRequestMessage(method, path);
    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("FORMFEED_API_KEY"));
    if (body is not null) request.Content = JsonContent.Create(body);
    var response = await http.SendAsync(request);
    var answer = await response.Content.ReadFromJsonAsync<JsonElement>();
    if (!response.IsSuccessStatusCode)
    {
        Console.Error.WriteLine($"{answer.GetProperty("title")}: {(answer.TryGetProperty("detail", out var d) ? d : "")}");
        Environment.Exit(1);
    }
    return answer;
}

// .NET has no CSV reader; this file has no quoted fields, so a split does. Otherwise: CsvHelper.
var lines = await File.ReadAllLinesAsync("attendees.csv");
var columns = lines[0].Split(',');
var items = lines.Skip(1).Select(line =>
{
    var values = line.Split(',');
    var data = new Dictionary<string, object>();
    for (var i = 0; i < columns.Length; i++) data[columns[i]] = values[i];
    // every field of a CSV is text: the template expects hours as a number
    data["hours"] = int.Parse(values[Array.IndexOf(columns, "hours")]);
    return new { data };
}).ToArray();

var queued = await Api(HttpMethod.Post, "renders/batch", new { template = "certificate", zip = true, items });
var id = queued.GetProperty("id").GetString();
Console.WriteLine($"{id} {queued.GetProperty("total")} queued");

// the job is always asynchronous: ask for it until it is no longer queued or processing
var job = queued;
while (job.GetProperty("status").GetString() is "queued" or "processing")
{
    await Task.Delay(2000);
    job = await Api(HttpMethod.Get, $"jobs/{id}", null);
}
Console.WriteLine($"{job.GetProperty("succeeded")}/{job.GetProperty("total")} rendered, {job.GetProperty("failed")} failed");

if (job.GetProperty("zip_url").GetString() is { } zip)
    await File.WriteAllBytesAsync("certificates.zip", await http.GetByteArrayAsync(zip));

Run it

It prints the job id and how many renders were queued, then 4/4 rendered, 0 failed, and certificates.zip lies next to the program with one PDF per row, 0001-document.pdf to 0004-document.pdf.

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.

The CSV

One header row, one line per document. The columns are the fields the template expects, so no mapping table is needed:

name,course,completed_on,hours
Erika Mustermann,Analytical Engines 101,2026-09-10,16
Max Mustermann,Analytical Engines 101,2026-09-10,16
Jane Doe,Print CSS for developers,2026-09-11,8
John Doe,Print CSS for developers,2026-09-11,8

What a batch does differently

• Always asynchronous. The answer comes back at once with the status queued. The renders run in the background, as many at a time as your plan allows.
• One template, many items. A top-level template applies to every item that does not name its own, so an item is usually just its data.
• Partly failed is a state. A job in which some items failed ends as partially_failed (failed when all did). succeeded and failed count them, and every item is a render of its own in items, with its own error.
• The zip is optional. Every item keeps its own download URL in items, which is what you want when the documents go to different people. zip: true adds one archive of the succeeded ones, each named 0001- and its item’s filename (document.pdf without one), in row order.
• Or a webhook. Instead of waiting, give the batch a webhook_url and let job.completed (or job.failed, when every item failed) tell you. For thousands of documents that is the better shape: no process waiting for minutes.

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 Certificate generator 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)
• Merge PDFs and add a password in C# (.NET)
• Async PDF rendering with a verified webhook in C# (.NET)

FAQ

How many rows can one job have?

Up to 1,000 items per batch. Split a longer list into several jobs; there is no limit on how many jobs you run, and each counts once against the rate limit. See async and batch.

What happens when one row is wrong?

When the template fails on a row’s data, that item fails and the others still render: the job ends as partially_failed and the failed item carries the error, so you can fix the row and render it again on its own. An item the API refuses outright (a request field it does not accept, or data that misses the template’s schema under validate_data) refuses the whole batch before anything is queued, and the problem names the item’s position.

Does a failed item cost units?

No. Only documents that were produced are counted.

Can I send a spreadsheet instead of writing code?

Yes: the app takes a CSV or Excel file, maps the columns to the template’s fields and runs the batch for you. The code here is for the same thing inside your own system.

Batch from a CSV in other languages

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