Packing slip PDFs in ASP.NET Core

Why packing slips become PDFs

A packing slip is printed and physically placed in a box. It exists to be paper. That is the difference between a document and a view of a database, and it is why warehouse and fulfilment teams keep asking for this.

What a packing slip has to carry

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

• the order reference, which the picker checks against the pick list
• each line with SKU, description and quantity shipped
• backordered lines shown explicitly rather than omitted
• the ship-to address as it will appear on the carton
• the carrier reference or tracking number where it exists

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/PackingSlipController.cs
using System.Net.Http.Json;

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

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

    [HttpGet("/packing-slips/{id}.pdf")]
    public async Task<IActionResult> Pdf(string id, CancellationToken ct)
    {
        var packingSlip = await _packingSlips.FindAsync(id, ct);
        var html = await _razor.RenderViewToStringAsync("PackingSlipPdf", packingSlip);

        // 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", $"packing-slip-{id}.pdf");
    }
}

The CSS that makes a packing slip page correctly

The layout problem specific to this document is that it must print correctly on the label and A5 stock warehouses actually use, not just on A4. These rules handle it.

/* Warehouses print on A5 and label stock, not A4. */
@page {
  size: A5;
  margin: 8mm;
}

.signature {
  break-inside: avoid;
  margin-top: auto;
}

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 packing slip lives afterwards

Rendering is the short part. The packing slip is thrown away when the box is opened. It has essentially no retention requirement, which is unusual on this list and worth knowing: the document does not need to be stored, only printed correctly the first time.

Getting the document right

• Check the order reference and quantities, which a picker checks against the physical goods.
• Generation is triggered by an order moving to fulfilment, 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 packing slip in other stacks.