unoplatform/uno · error · NotSupportedException

GenerateBitmap is not supported by this platform

Error message

GenerateBitmap is not supported by this platform

What it means

Thrown by SampleChooserViewModel.GenerateBitmap on the WebAssembly (__WASM__) build. RenderTargetBitmap rendering to a PNG file via BitmapEncoder is not available on WASM, so the method short-circuits with NotSupportedException. The method is compiled conditionally: on non-WASM it is async and performs the render; on WASM it throws immediately.

Source

Thrown at src/SamplesApp/SamplesApp.UnitTests.Shared/Controls/UITests/Presentation/SampleChooserViewModel.cs:1301

			var folder = await GetStorageFolderFromNameOrCreate(ct, folderName);

			if (_log.IsEnabled(LogLevel.Debug))
			{
				_log.Debug($"Output folder for tests: {folder.Path}");
			}
		}

		private
#if !__WASM__
			async
#endif
			Task GenerateBitmap(CancellationToken ct
			, Microsoft.UI.Xaml.Media.Imaging.RenderTargetBitmap targetBitmap
			, StorageFile file
			, FrameworkElement content)
		{
#if __WASM__
			throw new NotSupportedException($"GenerateBitmap is not supported by this platform");
#else
			var element = Owner.XamlRoot.Content;

			try
			{
				targetBitmap = new Microsoft.UI.Xaml.Media.Imaging.RenderTargetBitmap();

				await targetBitmap.RenderAsync(element).AsTask(ct);

				var pixels = await targetBitmap.GetPixelsAsync().AsTask(ct);

				using (var fileStream = await file.OpenAsync(FileAccessMode.ReadWrite).AsTask(ct))
				{
					var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, fileStream).AsTask(ct);

					encoder.SetPixelData(
						BitmapPixelFormat.Bgra8,
						BitmapAlphaMode.Ignore,

View on GitHub (pinned to 0418340488)

Solutions

1. Skip the bitmap-generation step on WASM — guard the caller with '#if !__WASM__' or a runtime platform check before invoking GenerateBitmap.
2. Run the screenshot flow on a non-WASM host (Skia desktop) where RenderTargetBitmap is supported.
3. If WASM screenshots are required, use the WASM HTML-capture/screenshot tooling instead of RenderTargetBitmap.

Example fix

// before
#if __WASM__
throw new NotSupportedException($"GenerateBitmap is not supported by this platform");
#else
... render ...
#endif
// after - guard the caller so the method is never entered on WASM
#if !__WASM__
await GenerateBitmap(ct, targetBitmap, file, content);
#else
_log.Information("Skipping bitmap generation on WASM.");
#endif

#if !__WASM__
await GenerateBitmap(ct, targetBitmap, file, content);
#else
_log.Information("Bitmap generation skipped on WASM.");
#endif

try { await GenerateBitmap(ct, targetBitmap, file, content); }
catch (NotSupportedException ex) when (ex.Message.Contains("GenerateBitmap is not supported"))
{ _log.Information(ex.Message); }

When it happens

Trigger: Running the sample-chooser screenshot/bitmap-generation flow under the WASM host. The build define __WASM__ routes execution to the throw branch.

Common situations: Test/CI harness targeting WASM that invokes screenshot generation; running the full SamplesApp on WASM where bitmap dumping is part of the test runner.

Related errors

• StepBackwardOneFrame is not supported
• StepForwardOneFrame is not supported
• retrieveFiles failed to find pending drag and drop data for
• Keyboard lock is not supported by this browser.
• BrowserHtmlElement: Element with id '${id}' not found.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/790310a7f126fa38. Report an issue: GitHub.