Implement EqualityComparer<T>.Create with key selector parameters by weitzhandler · Pull Request #125024 · dotnet/runtime

weitzhandler · 2026-03-01T05:57:45Z

Copilot

Pull request overview

Adds a new factory overload to System.Collections.Generic.EqualityComparer<T> that builds an EqualityComparer<T> from a key selector (and optional key comparer), enabling common “compare by property” scenarios (per #115797).

Changes:

Added EqualityComparer<T>.Create<TKey>(Func<T?, TKey?> keySelector, IEqualityComparer<TKey>? keyComparer = null) in CoreLib.
Added unit tests covering null keySelector and basic equality behavior with/without a custom key comparer.
Updated System.Collections reference assembly surface area to include the new overload.

Reviewed changes

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

File	Description
src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs	Introduces the new `Create<TKey>` overload and wires it through the existing delegate-based comparer implementation.
src/libraries/System.Collections/tests/Generic/Comparers/EqualityComparer.Tests.cs	Adds tests for the new overload’s argument validation and `Equals` behavior.
src/libraries/System.Collections/ref/System.Collections.cs	Updates the reference contract to expose the new API.

Copilot

Pull request overview

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

Copilot

Pull request overview

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

Copilot

Pull request overview

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

ViveliDuCh · 2026-04-10T06:12:05Z

Thanks for working on this! :)

I noticed that the API review also discussed adding a Comparer<T>.Create<TKey>() overload alongside this for symmetry. Would you consider including it as well?

Copilot

Pull request overview

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

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

…ash code test assertions - Remove null short-circuit in GetHashCode that bypassed keySelector, ensuring consistency with Equals which always passes null through keySelector (per reviewer feedback from eiriktsarpalis). - Add GetHashCode assertions to EqualityComparerCreate_KeySelectorUsed test to verify equal items produce equal hash codes. - Add GetHashCode assertions to EqualityComparerCreate_KeySelectorComparerUsed test including null-handling scenarios, verifying the equality/hash code contract holds for all cases. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- KeySelectorPassesNullToSelector: verifies null flows to keySelector in both Equals and GetHashCode (not short-circuited), with call counting. - KeySelectorReturnsNullKey: verifies correct behavior when keySelector maps an input to a null key (GetHashCode returns 0, Equals uses key-level null comparison). - KeySelectorNotHandlingNull_Throws: verifies NullReferenceException propagates when keySelector doesn't guard against null. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Copilot AI review requested due to automatic review settings March 1, 2026 05:57

github-actions Bot added the area-System.Collections label Mar 1, 2026

dotnet-policy-service Bot added the community-contribution Indicates that the PR has been added by a community member label Mar 1, 2026

weitzhandler force-pushed the eq-comparer-issue-115797 branch from 5ba9d15 to 55f807c Compare March 1, 2026 05:58

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weitzhandler mentioned this pull request Mar 1, 2026

[API Proposal]: EqualityComparer.Create from selecting a key property #115797

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Comment thread src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs Outdated

Comment thread src/libraries/System.Collections/tests/Generic/Comparers/EqualityComparer.Tests.cs Outdated

weitzhandler force-pushed the eq-comparer-issue-115797 branch from 55f807c to 6ccb5b5 Compare March 1, 2026 06:03

Copilot AI review requested due to automatic review settings March 1, 2026 06:09

weitzhandler force-pushed the eq-comparer-issue-115797 branch from 6ccb5b5 to 0ca8025 Compare March 1, 2026 06:09

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weitzhandler force-pushed the eq-comparer-issue-115797 branch from 0ca8025 to 02b5e71 Compare March 1, 2026 06:12

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Comment thread src/libraries/System.Collections/tests/Generic/Comparers/EqualityComparer.Tests.cs

Comment thread src/libraries/System.Collections/ref/System.Collections.cs Outdated

Comment thread src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs

weitzhandler force-pushed the eq-comparer-issue-115797 branch from 02b5e71 to 3588128 Compare March 1, 2026 06:35

weitzhandler requested a review from Copilot March 1, 2026 06:42

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Comment thread src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs

weitzhandler force-pushed the eq-comparer-issue-115797 branch from 3588128 to 34270f6 Compare March 1, 2026 06:58

weitzhandler marked this pull request as ready for review March 1, 2026 12:01

Copilot AI review requested due to automatic review settings March 1, 2026 12:01

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Comment thread src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs Outdated

Comment thread src/libraries/System.Collections/tests/Generic/Comparers/EqualityComparer.Tests.cs

This was referenced Mar 1, 2026

System.IO.Tests.Directory_Create_Tests.FileSystemWatcher_Directory_Create_InNestedDirectory fails with "Created event did not occur as expected" #103630

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Android System.Security.Cryptography test failure: Signing key verifies the CRL #119023

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eiriktsarpalis added the needs-author-action An issue or pull request that requires more info or actions from the author. label Apr 6, 2026

ViveliDuCh reviewed Apr 10, 2026

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Comment thread src/libraries/System.Collections/tests/Generic/Comparers/EqualityComparer.Tests.cs

dotnet-policy-service Bot added no-recent-activity and removed needs-author-action An issue or pull request that requires more info or actions from the author. no-recent-activity labels Apr 24, 2026

Copilot AI review requested due to automatic review settings April 24, 2026 11:29

weitzhandler force-pushed the eq-comparer-issue-115797 branch from 5d16bfc to 7bae40c Compare April 24, 2026 11:29

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Comment thread src/libraries/System.Collections/tests/Generic/Comparers/EqualityComparer.Tests.cs

Copilot AI review requested due to automatic review settings April 27, 2026 13:55

Copilot started reviewing on behalf of eiriktsarpalis April 27, 2026 13:56 View session

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eiriktsarpalis approved these changes Apr 27, 2026

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weitzhandler and others added 3 commits May 15, 2026 16:03

Implement EqualityComparer<T>.Create with key selector parameters

60fdd43

weitzhandler force-pushed the eq-comparer-issue-115797 branch from dc9ce64 to 2c90ea3 Compare May 15, 2026 15:04

build-analysis Bot mentioned this pull request May 16, 2026

browser-wasm linux Release LibraryTests queues timing out #117974

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Merge branch 'main' into eq-comparer-issue-115797

f797e8b

Copilot AI review requested due to automatic review settings May 18, 2026 13:14

eiriktsarpalis enabled auto-merge (squash) May 18, 2026 13:14

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eiriktsarpalis merged commit 7c254b3 into dotnet:main May 18, 2026
148 of 152 checks passed

build-analysis Bot mentioned this pull request May 18, 2026

[wasm] Tests failing with DirectoryNotFoundException trying to load test data #128293

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dotnet-maestro Bot mentioned this pull request May 19, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#6681

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weitzhandler commented Mar 1, 2026

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ViveliDuCh commented Apr 10, 2026

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