C#: Improve the query cs/gethashcode-is-not-defined.

[michaelnebel]

Even though there is a general compiler warning CS0659 for all types overriding Equals but not GetHashCode - this query only targets expressions of types, where it is more likely to have an impact that GetHashCode() is not overridden.

[michaelnebel]

DCA looks good; We found a couple of extra results, but the query also performs slightly worse.

[Copilot]

Pull Request Overview

Enhance the cs/gethashcode-is-not-defined query to catch more hashing usages across both method calls and indexers, backed by new framework definitions and expanded tests.

• Extend the main QL predicate (usesHashing) to include more methods and indexer accesses.
• Add support for IReadOnlyDictionary and HashSet in the framework library.
• Update test suite and change notes to reflect the broader detection.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
csharp/ql/src/Likely Bugs/HashedButNoHash.ql	Updated imports and predicates to cover additional scenarios, renamed and refactored predicates (usesHashing, dictionary, hashSet, hashStructure).
csharp/ql/lib/semmle/code/csharp/frameworks/system/collections/Generic.qll	Added IReadOnlyDictionary and HashSet class definitions.
csharp/ql/src/change-notes/2025-05-15-gethashcode-is-not-defined.md	Added change note documenting the precision improvements.
csharp/ql/test/query-tests/Likely Bugs/HashedButNoHash/HashedButNoHash.qlref	Adjusted test metadata to use query: and postprocess: directives.
csharp/ql/test/query-tests/Likely Bugs/HashedButNoHash/HashedButNoHash.expected	Expanded expected alerts to cover the new hashing calls.
csharp/ql/test/query-tests/Likely Bugs/HashedButNoHash/HashedButNoHash.cs	Added more hashing-related calls (Contains, Remove, TryAdd, TryGetValue) to the test case.

Comments suppressed due to low confidence (3)

csharp/ql/src/Likely Bugs/HashedButNoHash.ql:29

• The Javadoc comment refers to parameter c but the predicate declares parameter t; consider updating the comment to match the actual parameter name for clarity.

/** Holds if `c` is a hashset type. */

csharp/ql/src/Likely Bugs/HashedButNoHash.ql:14

• The import path uses an uppercase 'Collections' segment, which may be inconsistent with the lowercase directory naming used elsewhere; please verify the correct casing for the module path.

import semmle.code.csharp.frameworks.system.Collections

csharp/ql/src/change-notes/2025-05-15-gethashcode-is-not-defined.md:1

• [nitpick] The frontmatter defines only category; to maintain consistency with other change notes, consider adding a title field or confirming this matches the existing style conventions.

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