Strip partial modifier from generated COM interface method stubs

[Copilot]

Description

ComInterfaceGenerator and VtableIndexStubGenerator copy method modifiers from user-declared interface methods into generated stub code. The new keyword and accessibility modifiers were already stripped, but partial was not — producing invalid C# when the user declares partial methods on a [GeneratedComInterface] interface.

Changes:

• ComInterfaceGenerator.cs: Filter SyntaxKind.PartialKeyword alongside existing NewKeyword filter in CalculateStubInformation
• VtableIndexStubGenerator.cs: Add PartialKeyword filter to modifier stripping in CalculateStubInformation
• Compiles.cs: Add regression test confirming partial method modifiers don't produce invalid generated code

Note: LibraryImportGenerator and DownlevelLibraryImportGenerator are intentionally unchanged — they generate partial method implementations where partial must be preserved.

// User code that previously caused invalid generated output:
[GeneratedComInterface]
[Guid("...")]
internal partial interface ICalculator
{
    int Add(int a, int b);
    public partial int Subtract(int a, int b) => 0;
    public partial int Subtract(int a, int b);
}

[Copilot]

Pull request overview

This PR updates the COM interface source generators to avoid emitting invalid C# when user-declared COM interface methods include the partial modifier, by stripping partial from copied method modifiers and adding a regression compilation test.

Changes:

• Strip SyntaxKind.PartialKeyword from copied method modifiers in ComInterfaceGenerator and VtableIndexStubGenerator.
• Add a regression test ensuring partial method modifiers on [GeneratedComInterface] methods don’t break generated code compilation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/Compiles.cs	Adds a new compilation regression test for partial method modifiers on COM interface methods.
src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs	Filters out partial from method modifiers when constructing the syntax template for generated stubs.
src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComInterfaceGenerator.cs	Filters out partial (alongside existing new filtering) from method modifiers used in generated stubs.

[Copilot]

virtual is not a valid modifier on an interface declaration, so this test source will fail to compile before the generator runs. Remove virtual from both IComInterface declarations (keep partial).

Suggested change

	internal virtual partial interface IComInterface
	{
	void Method();
	public partial void PartialMethod();
	}
	internal virtual partial interface IComInterface
	internal partial interface IComInterface
	{
	void Method();
	public partial void PartialMethod();
	}
	internal partial interface IComInterface

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

The PR description mentions that both ComInterfaceGenerator and VtableIndexStubGenerator already strip the new modifier, but in this code path VtableIndexStubGenerator only strips accessibility (and now partial). Either update the PR description, or (if intended) extend this modifier filter to also remove SyntaxKind.NewKeyword for consistency with ComInterfaceGenerator.

[github-actions]

🤖 Copilot Code Review — PR #126719

Note

This review was generated by Copilot (Claude Opus 4.6). A GPT-5.4 sub-agent was launched in parallel but did not return within the timeout window; this review reflects a single-model analysis.

Holistic Assessment

Motivation: Justified. When a user declares a partial method on a [GeneratedComInterface] or [VirtualMethodIndex] interface, the partial modifier leaked into generated explicit interface implementation stubs, producing invalid C#. This is a real bug with a clear repro scenario.

Approach: Correct and consistent with established patterns. The NewKeyword was already being stripped in ComInterfaceGenerator for the same class of reasons; extending this to PartialKeyword in both generators is the minimal, targeted fix.

Summary: ✅ LGTM. The fix is correct, minimal, and well-tested for the primary code path. One minor test coverage observation below (non-blocking).

---

Detailed Findings

✅ Correctness — Partial modifier stripping is correct

The generated methods are explicit interface implementations (via .WithExplicitInterfaceSpecifier(...) in VirtualMethodPointerStubGenerator.PrintGeneratedSource), where a partial modifier is invalid C#. Both ComInterfaceGenerator.cs (line 366) and VtableIndexStubGenerator.cs (line 271) now correctly filter PartialKeyword before passing modifiers to ContainingSyntax. The LibraryImportGenerator and DownlevelLibraryImportGenerator correctly do not strip partial, since they generate partial method bodies (where partial is required).

✅ Test — The test validates the fix for ComInterfaceGenerator

The new PartialMethodModifierOnComInterfaceMethodCompiles test provides a realistic scenario with a partial interface method declared across two partial interface declarations, verifying the source generator produces valid output.

💡 Test coverage — VtableIndexStubGenerator path not directly tested

The fix modifies both ComInterfaceGenerator and VtableIndexStubGenerator, but the new test only exercises the ComInterfaceGenerator path (via VerifyComInterfaceGenerator.VerifySourceGeneratorAsync). A parallel test using VerifyVTableGenerator.VerifySourceGeneratorWithAncillaryInteropAsync with a [VirtualMethodIndex] partial method would provide coverage for the second code path. This is non-blocking — the code change is clearly correct by inspection — but worth considering as a follow-up.

💡 Pre-existing inconsistency — NewKeyword not filtered in VtableIndexStubGenerator (not introduced by this PR)

ComInterfaceGenerator filters both NewKeyword and PartialKeyword, while VtableIndexStubGenerator only filters PartialKeyword. Since new can appear on interface methods (when hiding a base interface method), the same issue could theoretically occur with VtableIndexStubGenerator and new. This is pre-existing and out of scope for this PR.

Generated by Code Review for issue #126719 · ◷