dotnet/AspNetCore.Docs · error · Exception

QR code size must be less than {MaxQrSize}.

Error message

QR code size must be less than {MaxQrSize}.

What it means

The Blazor web-worker QR sample guards `Generate(text, qrSize)` with `if (qrSize >= MaxQrSize) throw` where MaxQrSize is 20. qrSize is the pixels-per-module passed to QRCoder's `GetGraphic`; large values blow up worker memory and render time, so the sample caps it.

Source

Thrown at aspnetcore/blazor/blazor-with-dotnet-on-web-workers.md:285

`Workers/QRGenerator.razor.cs`:

```csharp
using System.Runtime.InteropServices.JavaScript;
using System.Runtime.Versioning;
using QRCoder;

[SupportedOSPlatform("browser")]
public partial class QRGenerator
{
    private static readonly int MaxQrSize = 20;

    [JSExport]
    internal static byte[] Generate(string text, int qrSize)
    {
        if (qrSize >= MaxQrSize)
        {
            throw new Exception($"QR code size must be less than {MaxQrSize}.");
        }

        var generator = new QRCodeGenerator();
        QRCodeData data = generator.CreateQrCode(text, QRCodeGenerator.ECCLevel.Q);
        var qrCode = new BitmapByteQRCode(data);
    
        return qrCode.GetGraphic(qrSize);
    }
}
```

Create a matching Razor component file (`.razor`) to act as an empty stub so that the build packs the worker script alongside the component assets:

`Workers/QRGenerator.razor`:

```razor
// dummy file to let blazor handle Worker.razor.js file loading
```

View on GitHub (pinned to c67a80103a)

Solutions

1. Pass a size strictly less than 20 (e.g. 10–15 for typical QR codes).
2. Clamp the caller-supplied value before invoking the worker: `size = Math.min(size, 19)`.
3. Validate input in the message handler and surface a friendly error to the UI rather than letting the throw propagate.

Example fix

// before
const bytes = assemblyExports.QRGenerator.Generate(text, 32);

// after
const size = Math.min(requestedSize, 19);
const bytes = assemblyExports.QRGenerator.Generate(text, size);

function safeQrSize(requested: number, maxExclusive = 20): number {
  if (!Number.isFinite(requested) || requested <= 0) throw new RangeError('qrSize must be a positive number');
  return Math.min(Math.floor(requested), maxExclusive - 1);
}

function isValidQrSize(size: number, maxExclusive = 20): boolean {
  return Number.isInteger(size) && size > 0 && size < maxExclusive;
}

// Worker side already throws; guard on the caller:
try {
  await postToWorker({ command: 'generateQR', text, size: safeQrSize(size) });
} catch (err) {
  if (/QR code size/.test(err.message)) showUserError('Pick a smaller QR size.');
}

When it happens

Trigger: Calling the exported `QRGenerator.Generate(text, size)` with `size >= 20`. The check is `>=`, so the maximum allowed value is 19.

Common situations: Caller passes a default or guessed size (e.g. 25 or 32); UI control lets the user pick any number; size derived from device pixel ratio without clamping.

Related errors

• Unknown command: ${e.data.command}
• worker exports not loaded
• Validation failed. Status Code: {response.StatusCode}
• Must provide a non-empty value for the "src" attribute.
• When calling ko.fromJS, pass the object you want to convert.

AI-assisted analysis of dotnet/AspNetCore.Docs@c67a80103a (2026-08-13). Data as JSON: /api/errors/682b223737dcb283. Report an issue: GitHub.