C#: Fix issue with partial method extraction.

[michaelnebel]

It turns out that our current implementation for extracting partial methods in some cases might extract multiple method bodies.
An example:

partial class TwoPartClass
{
    public partial object M(object obj);
}
partial class TwoPartClass
{
    public partial object M(object obj)
    {
        return obj;
    }
}

In this case the following happens in the extractor:

• The type TwoPartClass is visited and the members are extracted. However, only the partial definition is listed as a member (the one without the body).
• During extraction of the symbol representing the partial definition, we also extract the "body" of the partial declaration.
• During the extraction of the partial declaration body we encounter and extract the parameter symbol corresponding to obj (as the parameter is accessed).
• Extracting the parameter symbol obj as a side effect also extracts its parent, which is the symbol representing the partial declaration of M.
• Even though method extraction is done using caching, the symbol for the partial definition and partial declaration are different, which means that the symbol representing the partial declaration is fully extracted as well.
• The extracted tuple ID for the partial implementation and declaration are identical, which means that the extraction is collapsed into a single entity.
• Extracting the body of the partial implementation, will yet again extract the body of the partial implementation.

As a result of the above, the body might be extracted twice.

In this PR we skip the extraction of the partial definition and only extract the partial declaration (if one exists).

Review on a commit by commit basis is recommended.

[michaelnebel]

DCA looks good.

• Performance doesn't appear to be affected.
• There are a few more extraction errors in the msbuild project, but these are compilation errors (I spot checked and one of them appears to be related to the use of labels) and not directly related to the change in this PR.

[Copilot]

Pull request overview

Fixes a C# extractor issue where partial members (notably partial methods) could have their bodies extracted multiple times due to symbol/caching interactions between defining vs implementing declarations.

Changes:

• Prefer extracting the body-declaring symbol for methods/properties/events (via new GetBodyDeclaringSymbol helpers) to avoid duplicate extraction for partial members.
• Refactor body detection (Block/ExpressionBody) into cached helpers and use HasBody for accessors/event accessors.
• Update/add library tests and expectations, including a new dataflow regression test involving a partial method body.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs	Adds GetBodyDeclaringSymbol helpers (methods/properties/events).
csharp/extractor/Semmle.Extraction.CSharp/Entities/OrdinaryMethod.cs	Creates ordinary method entities from the body-declaring symbol.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Method.cs	Uses Symbol for NumberOfLines after symbol selection changes.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Property.cs	Creates property entities from the body-declaring symbol; updates accessor extraction to use Symbol.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Indexer.cs	Creates indexer entities from the body-declaring symbol; updates accessor extraction to use Symbol.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Event.cs	Creates event entities from the body-declaring symbol; updates accessor extraction to use Symbol.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Base/CachedSymbol.cs	Adds cached Block/ExpressionBody and HasBody helpers.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Accessor.cs	Uses HasBody when deciding whether to mark accessors as compiler-generated.
csharp/extractor/Semmle.Extraction.CSharp/Entities/EventAccessor.cs	Uses HasBody when deciding whether to mark event accessors as compiler-generated.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Constructor.cs	Renames/refactors synthetic-body logic (MakeSyntheticBody) and uses HasBody.
csharp/ql/lib/change-notes/2026-02-23-partial-extraction-fix.md	Adds changelog entry for the extraction fix.
csharp/ql/test/library-tests/partial/Partial.cs	Extends partial-member test input with a partial method with body and parameters.
csharp/ql/test/library-tests/partial/PartialMethodBody.ql	Extends test query to assert body-count is 0/1 (detect duplicates).
csharp/ql/test/library-tests/dataflow/methods/Methods.cs	Adds a partial-method dataflow scenario for regression coverage.
csharp/ql/test/library-tests/dataflow/methods/MethodFlow.ql	Adds a path-problem query to validate flow through a partial method body.
csharp/ql/test/library-tests/*.expected	Updates expected outputs to reflect the new extraction behavior and new tests.

[Copilot]

Block/ExpressionBody use vBlock ??= / vExpressionBody ??= which won’t memoize a null result, so symbols without bodies will re-scan DeclaringSyntaxReferences on every access. If the intent is to cache the lookup, consider caching the computed state separately (or using Lazy<T?>) so null is memoized too.

Suggested change

	public BlockSyntax? Block => vBlock ??= GetBlock(Symbol);
	private ExpressionSyntax? vExpressionBody;
	public ExpressionSyntax? ExpressionBody => vExpressionBody ??= GetExpressionBody(Symbol);
	private bool vBlockComputed;
	public BlockSyntax? Block
	{
	get
	{
	if (!vBlockComputed)
	{
	vBlock = GetBlock(Symbol);
	vBlockComputed = true;
	}
	return vBlock;
	}
	}
	private ExpressionSyntax? vExpressionBody;
	private bool vExpressionBodyComputed;
	public ExpressionSyntax? ExpressionBody
	{
	get
	{
	if (!vExpressionBodyComputed)
	{
	vExpressionBody = GetExpressionBody(Symbol);
	vExpressionBodyComputed = true;
	}
	return vExpressionBody;
	}
	}