CSharpSyntaxGenerator does not support adding interface members with a default implementation

[jkoritzinsky]

Version Used: 3.9.0-3.final. The failing code is in main at

roslyn/src/Workspaces/CSharp/Portable/CodeGeneration/CSharpSyntaxGenerator.cs

Lines 771 to 796 in 17facfa

	case SyntaxKind.MethodDeclaration:
	return ((MethodDeclarationSyntax)member)
	.WithModifiers(default)
	.WithSemicolonToken(SyntaxFactory.Token(SyntaxKind.SemicolonToken))
	.WithBody(null);
	case SyntaxKind.PropertyDeclaration:
	var property = (PropertyDeclarationSyntax)member;
	return property
	.WithModifiers(default)
	.WithAccessorList(WithoutBodies(property.AccessorList));
	case SyntaxKind.IndexerDeclaration:
	var indexer = (IndexerDeclarationSyntax)member;
	return indexer
	.WithModifiers(default)
	.WithAccessorList(WithoutBodies(indexer.AccessorList));
	// convert event into field event
	case SyntaxKind.EventDeclaration:
	var ev = (EventDeclarationSyntax)member;
	return this.EventDeclaration(
	ev.Identifier.ValueText,
	ev.Type,
	Accessibility.NotApplicable,
	DeclarationModifiers.None);

Steps to Reproduce:

1. Given a SyntaxGenerator named generator as part of a code-fix running in C#, an IMethodSymbol named method, and a SyntaxNode named declaration that represents an interface, run the following snippet:

SyntaxNode methodDecl = generator.MethodDeclaration(method);
methodDecl = generator.WithModifiers(methodDecl, generator.GetModifiers(declaration).WithIsAbstract(false));
methodDecl = generator.WithStatements(methodDecl, generator.ThrowStatement(generator.NullLiteralExpression()));
declaration = generator.AddMembers(declaration, methodDecl);

Expected Behavior:

The generated code in declaration will have a method based on method that has the following body:

{
    throw null;
}

Actual Behavior:

The generated code will have an interface member declaration with no body.

Based on the code in Roslyn linked above, this bug will be hit for properties, indexers, and event declarations as well.

This is blocking providing the code fixes as part of dotnet/runtime#41529

[CyrusNajmabadi]

This is about SyntaxGenerator not SourceGenerator.

[jkoritzinsky]

Sorry about that. Fixed the content of the issue since you already fixed the title.

[jinujoseph]

Assigning to Cyrus to review the API

[CyrusNajmabadi]

This is challenging as we've always supported generating interface members by taking a normal member and just adding it to an interface. We toss things like the bodies in this case. So it's very likely that code exists that depends on that behavior. So it could be extremely problematic to change that now.

It's not clear what a viable approach would be here except maybe to allow AddMembers to specify some new behaviors here that can be opted into.