Generate a QR code in C#

QRCoder

dotnet add package QRCoder and the whole flow is generator → data → renderer:

using QRCoder;

using var generator = new QRCodeGenerator();
var qrData = generator.CreateQrCode("https://example.com", QRCodeGenerator.ECCLevel.Q);
byte[] png = new PngByteQRCode(qrData).GetGraphic(20);
File.WriteAllBytes("qr.png", png);

Two details that save real debugging time:

• Use PngByteQRCode, not the legacy QRCode class. The legacy renderer draws through System.Drawing, which is Windows-only on modern .NET — it throws at runtime on Linux containers. PngByteQRCode writes the PNG itself and runs everywhere.
• GetGraphic(20) is pixels per module, not total size. A version 2 code (25 modules plus the quiet zone) at 20 px/module lands around 660 px.

ECCLevel.L/M/Q/H are the standard error-correction levels. For print, use SvgQRCode from the same package — it returns an SVG string, and vector is what print needs.

An ASP.NET endpoint

Minimal API version — generation is fast enough to do per-request, and the cache header makes repeats free:

app.MapGet("/qr", (string data) =>
{
    using var generator = new QRCodeGenerator();
    var qrData = generator.CreateQrCode(data, QRCodeGenerator.ECCLevel.M);
    return Results.Bytes(new PngByteQRCode(qrData).GetGraphic(16), "image/png");
});

Add Cache-Control: public, max-age=31536000, immutable if the mapping from input to image never changes — which for a QR code it does not.

The zero-dependency route: HttpClient

For styled codes (colours, module and eye shapes) or typed, validated payloads (/api/v1/wifi, /api/v1/upi, /api/v1/vcard), skip the package and call the keyless API:

using var http = new HttpClient();
var target = "https://example.com/sale?src=poster";
var url = $"https://useqr.app/api/v1/qr?data={Uri.EscapeDataString(target)}&size=1024";
await File.WriteAllBytesAsync("qr.png", await http.GetByteArrayAsync(url));

Uri.EscapeDataString is the load-bearing call — an unescaped & in the payload starts a new query parameter and truncates the data silently. Keep credentials (WiFi passwords, personal data) out of any remote call; generate those locally with QRCoder.

Verify styled output

Decode what you encode before it ships:

using System.Net.Http.Json;

var report = await http.GetFromJsonAsync<VerifyReport>(
    $"https://useqr.app/api/v1/verify?data={Uri.EscapeDataString(target)}&color=6366f1");
if (report is not { Scannable: true })
    throw new InvalidOperationException(string.Join("; ", report!.Issues));

record VerifyReport(bool Scannable, string[] Issues);

The endpoint renders, rasterises and reads the code back with a real decoder — the decode-verify loop in one GET. How QRCoder sits next to ZXing and friends is covered in QR code libraries compared; the JVM equivalent of this page is Java.

FAQ

What is the best C# library for QR codes?

QRCoder. Use its PngByteQRCode renderer, which has no System.Drawing dependency and therefore works on Linux and in containers.

Why does my QRCoder code crash on Linux?

You are using the legacy QRCode renderer, which needs System.Drawing — Windows-only on modern .NET. Switch to PngByteQRCode or SvgQRCode.

What does GetGraphic(20) mean?

Pixels per module, not total image size. Multiply by the module count of your code's version, plus the quiet zone, to predict output dimensions.

How do I return a QR code from ASP.NET?

Results.Bytes with the PngByteQRCode output and an image/png content type in a minimal API route. Add an immutable cache header since output is deterministic.