Ticket PDFs in ASP.NET Core

Why tickets become PDFs

A ticket has to work at a gate with no signal, on a phone that may be offline, or on paper. Anything requiring a live request fails at exactly the wrong moment. That is the difference between a document and a view of a database, and it is why event organisers and travel operators keep asking for this.

What a ticket has to carry

Before any of the code below matters, the template has to produce a document that is actually a ticket. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the scannable code, which is the ticket; everything else is decoration
• the event or journey, with date and time including timezone
• the seat, zone or class where the entitlement is specific
• the holder name where the ticket is non-transferable
• the terms of entry, which are what a gate refusal is justified by

Turning your Razor template into a document

You already have the markup. Await _razorRenderer.RenderViewToStringAsync("Invoice", model) gives you it as a string, which is the only input the render needs. Rendering a Razor view to a string outside the MVC pipeline needs an explicit ActionContext, which is why most projects end up with a small RazorViewToStringRenderer service. Write it once and inject it.

The ASP.NET Core implementation

IHttpClientFactory ships with the runtime, so this adds no dependency. The render happens in an action returning FileStreamResult, and you return File(stream, "application/pdf").

// Controllers/TicketController.cs
using System.Net.Http.Json;

[ApiController]
public class TicketController : ControllerBase
{
    private readonly IHttpClientFactory _factory;
    private readonly RazorViewToStringRenderer _razor;

    public TicketController(IHttpClientFactory factory, RazorViewToStringRenderer razor)
        => (_factory, _razor) = (factory, razor);

    [HttpGet("/tickets/{id}.pdf")]
    public async Task<IActionResult> Pdf(string id, CancellationToken ct)
    {
        var ticket = await _tickets.FindAsync(id, ct);
        var html = await _razor.RenderViewToStringAsync("TicketPdf", ticket);

        // From the factory. new HttpClient() per request exhausts sockets under
        // load, and the symptom only appears in production.
        var client = _factory.CreateClient("pdfpipe");

        var upstream = await client.PostAsJsonAsync("https://api.pdfpipe.xyz/v1/pdf", new
        {
            html,
            options = new { format = "A4", printBackground = true },
        }, ct);

        upstream.EnsureSuccessStatusCode();

        var stream = await upstream.Content.ReadAsStreamAsync(ct);
        return File(stream, "application/pdf", $"ticket-{id}.pdf");
    }
}

The CSS that makes a ticket page correctly

The layout problem specific to this document is that a barcode or QR code must render at a size and contrast a scanner can actually read after printing. These rules handle it.

/* The code has to survive printing. Keep it black on white at a
   size a scanner can resolve, and never let it break across a page. */
.barcode {
  break-inside: avoid;
  image-rendering: pixelated;
  min-width: 45mm;
  filter: none;
}

@page { size: A6 landscape; margin: 5mm; }

What goes wrong in ASP.NET Core

new HttpClient() per request exhausts sockets under sustained load, because each disposed handler leaves a connection in TIME_WAIT. Inject IHttpClientFactory. This is the most common .NET HTTP mistake that only appears in production.

What people try first

Most ASP.NET Core projects reach for PuppeteerSharp, which downloads and supervises a Chromium build from inside your own process, or QuestPDF, which is a fluent layout API rather than a way to render HTML you already have. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the ticket lives afterwards

Rendering is the short part. A ticket is worthless the moment the event ends, so nothing needs to survive it except the record that it was scanned. That record lives in your database, not in the PDF.

Getting the document right

• Check the code itself. A ticket that will not scan is worthless no matter how it looks.
• Generation is triggered by a booking being confirmed, so size the timeout for that path rather than for a health check.
• Volume arrives in bursts, so queue the render rather than doing it inside the request that triggered it.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other ASP.NET Core documents, and the same ticket in other stacks.