C# 14: Support for partial event declarations.

[michaelnebel]

In this PR we make support for partial event declarations.

That is, we support event declarations like.

public partial class Partial
{
    public partial event EventHandler MyEvent;
}

public partial class Partial
{
    public partial event EventHandler MyEvent {
        add { ... } 
        remove { ... } 
    }
}

The default add/remove event accessors from the declaring declaration are not extracted. Only the explicit add/remove from the implementation declaration are extracted. Note that a partial event always need to have an implementation declaration.

[michaelnebel]

DCA looks good

• Performance is unaffected.
• It looks like the only project that is impacted is dotnet/msbuild, which now has fewer extraction errors.

[Copilot]

Pull request overview

This PR adds support for C# 14 partial event declarations by ensuring that only accessors from the implementation declaration are extracted, not from the declaring declaration. The implementation follows the established pattern used for partial properties and indexers.

Changes:

• Added extractor support for partial events using BodyDeclaringSymbol pattern (similar to partial properties/indexers)
• Added NeedsPopulation override in EventAccessor to skip extraction of accessors from partial declaring declarations
• Added comprehensive test coverage for partial events including AST, accessor detection, and dispatch tests

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
csharp/extractor/Semmle.Extraction.CSharp/Entities/Event.cs	Implements BodyDeclaringSymbol and ReportingLocation to use implementation part for partial events; gets accessors from implementation declaration only; modernizes null check pattern
csharp/extractor/Semmle.Extraction.CSharp/Entities/EventAccessor.cs	Overrides NeedsPopulation to skip extraction of accessors from partial declaring declarations
csharp/ql/test/library-tests/partial/Partial.cs	Adds test cases for partial event declarations
csharp/ql/test/library-tests/partial/PartialEvents.ql	New query to test isPartial() predicate on events
csharp/ql/test/library-tests/partial/PartialEvents.expected	Expected output for partial events test
csharp/ql/test/library-tests/partial/PartialAccessors.expected	Updated to include partial event accessors
csharp/ql/test/library-tests/partial/Partial1.expected	Updated with partial event entries
csharp/ql/test/library-tests/partial/PrintAst.expected	Updated AST output showing partial event structure
csharp/ql/test/library-tests/partial/PartialProperties.expected	Line number updates due to added code
csharp/ql/test/library-tests/partial/PartialMethodBody.expected	Line number updates due to added code
csharp/ql/test/library-tests/partial/PartialIndexers.expected	Line number updates due to added code
csharp/ql/test/library-tests/partial/PartialConstructors.expected	Line number updates due to added code
csharp/ql/test/library-tests/partial/Partial2.expected	Line number updates due to added code
csharp/ql/test/library-tests/partial/MethodIsPartial.expected	Line number updates due to added code
csharp/ql/test/library-tests/dispatch/ViableCallable.cs	Adds test cases for partial event dispatch
csharp/ql/test/library-tests/dispatch/GetADynamicTarget.expected	Updated with partial event accessor dispatch targets
csharp/ql/test/library-tests/dispatch/CallGraph.expected	Updated call graph with partial event accessors
csharp/ql/lib/change-notes/2026-02-16-partial-events.md	Change note documenting C# 14 partial events support