Unexpected `System.Exception` thrown by source generator; reported through telemetry

[sharwell]

One or more of the following lines are throwing System.Exception at a location where exceptions are not expected:

uno/src/SourceGenerators/Uno.UI.SourceGenerators/DependencyObject/DependencyPropertyGenerator.cs

Lines 48 to 53 in 574a441

	_dependencyObjectSymbol = comp.GetTypeByMetadataName(XamlConstants.Types.DependencyObject)
	?? throw new Exception("Unable to find " + XamlConstants.Types.DependencyObject);
	_generatedDependencyPropertyAttributeSymbol = comp.GetTypeByMetadataName("Uno.UI.Xaml.GeneratedDependencyPropertyAttribute")
	?? throw new Exception("Unable to find Uno.UI.Xaml.GeneratedDependencyPropertyAttribute");
	_dependencyPropertyChangedEventArgsSymbol = comp.GetTypeByMetadataName("Windows.UI.Xaml.DependencyPropertyChangedEventArgs")
	?? throw new Exception("Unable to find Windows.UI.Xaml.DependencyPropertyChangedEventArgs");

This was identified in error telemetry at Microsoft, and I'm happy to take the time to provide additional details here because you've chosen to make your project open source. 👍

I recommend resolving this issue in one of the following ways:

1. If this issue only occurs when there are other compiler errors present in the user code, it would be acceptable to silently disable the source generator in this case. The source generator would become active again when the user corrects the code.
2. If this issue can occur where users might not know how to resolve the issue, rather than throw an exception the generator can use the ReportDiagnostic API to report a user-visible error specific to the scenario.

[jeromelaban]

Thanks Sam! Historically, this code was not running inside the compiler (it was through uno.sourcegenerationtasks) and was not telemetry-worthy :)

We'll be migrating to remove those exceptions when running in the roslyn pipeline, so it'll hopefully cleanup your traces over time and provide a better user experience ;)

Thanks again!

[Youssef1313]

I think I also introduced a similar issue in #6760. I did that because throwing was very common in Uno generators so I was blindly consistent.

I expect too many places to be throwing exceptions in generators, as @jeromelaban said.

[anpin]

I think I'm hitting this too in wild now trying to build a WinUI net6.0 sample with Prism PrismLibrary/Prism#2721

CSC warning CS8785: Generator 'DependencyPropertyGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'Exception' with message 'Unable to find Uno.UI.Xaml.GeneratedDependencyPropertyAttribute' [D:\blav-app\modules\Prism\e2e\Uno.WinUI\src\ModuleA\ModuleA.csproj]

binlog -> msbuild.zip

[Youssef1313]

@anpin I think you need to file a separate issue with a repro.

[Youssef1313]

For XamlCodeGenerator, we catch exceptions here:

uno/src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlCodeGeneration.cs

Lines 467 to 472 in fb32a4c

	catch (Exception e)
	{
	TrackGenerationFailed(e, stopwatch.Elapsed);
	return ProcessParsingException(e);
	}

and report them as diagnostics:

uno/src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlCodeGeneration.cs

Lines 520 to 528 in fb32a4c

	foreach (var exception in Flatten(e))
	{
	var diagnostic = Diagnostic.Create(
	XamlCodeGenerationDiagnostics.GenericXamlErrorRule,
	GetExceptionFileLocation(exception),
	exception.Message);
	_generatorContext.ReportDiagnostic(diagnostic);
	}

For DependencyPropertyGenerator, this is now an internal generator and was refactored to IIncrementalGenerator. It no longer throws exceptions with the refactoring, I think.

@sharwell Have you recently seen any errors from Uno generators in telemetry? I think this could be already fixed and can be closed. If you are able to confirm, that would be really great!