Optimize ImmutableHashSet<T>.IsProperSubsetOf to avoid unnecessary allocations

[aw0lid]

Part of #127279

Summary

ImmutableHashSet<T>.IsProperSubsetOf always creates a new intermediate HashSet<T> for the other collection, leading to avoidable allocations and GC pressure, especially for large datasets

Optimization Logic

• O(1) Pre-Scan: Immediately returns false if other is an ICollection with a smaller or equal Count. By performing this validation upfront, the need for tracking variables like matches and extraFound is eliminated, as any complete match is now mathematically guaranteed to be a proper subset.

• Fast-Path Pattern Matching: Detects ImmutableHashSet<T> and HashSet<T> to bypass intermediate allocations.

• Comparer Guard: Validates EqualityComparer compatibility before triggering fast paths to ensure logical consistency.

• Short-Circuit Validation: Re-validates Count within specialized paths for an immediate exit before $O(n)$ enumeration.

• Don't repeat your self: reused SetEqualsWithHashset and SetEqualsWithImmutableHashset methods to avoid code duplication while ensuring we leverage the $O(1)$ lookup efficiency when other is a Hashset<T>.

• Zero-Allocation Execution: Direct iteration over compatible collections, eliminating the costly new HashSet<T>(other) fallback.

• Deferred fallback: Reserves the expensive allocation solely for general IEnumerable types.

Click to expand Benchmark Source Code

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Order;usingBenchmarkDotNet.Running;usingSystem;usingSystem.Collections.Generic;usingSystem.Collections.Immutable;usingSystem.Linq;namespaceImmutableHashSetBenchmarks{[MemoryDiagnoser][Orderer(SummaryOrderPolicy.FastestToSlowest)][RankColumn]publicclassImmutableHashSetIsProperSubsetOfBenchmark{privateImmutableHashSet<int>_sourceSet=null!;privateImmutableHashSet<int>_immutableLarger=null!;privateHashSet<int>_bclHashSetLarger=null!;privateList<int>_listLarger=null!;privateint[]_arrayLarger=null!;privateImmutableHashSet<int>_immutableSmaller=null!;privateImmutableHashSet<int>_immutableSameCount=null!;privateHashSet<int>_bclHashSetLargerDiffComparer=null!;privateList<int>_listWithDuplicatesButProper=null!;privateImmutableHashSet<int>_emptySource=null!;privateList<int>_listSameElementsWithDuplicates=null!;[Params(100000)]publicintSize{get;set;}[GlobalSetup]publicvoidSetup(){varelements=Enumerable.Range(0,Size).ToList();varlargerElements=Enumerable.Range(0,Size+10).ToList();varsmallerElements=Enumerable.Range(0,Size-10).ToList();varreverseComparer=newReverseComparer<int>();_sourceSet=ImmutableHashSet.CreateRange(elements);_immutableLarger=ImmutableHashSet.CreateRange(largerElements);_bclHashSetLarger=newHashSet<int>(largerElements);_listLarger=largerElements;_arrayLarger=largerElements.ToArray();_immutableSmaller=ImmutableHashSet.CreateRange(smallerElements);_immutableSameCount=ImmutableHashSet.CreateRange(elements);_bclHashSetLargerDiffComparer=newHashSet<int>(largerElements,reverseComparer);_listWithDuplicatesButProper=elements.Concat(new[]{Size+1,Size+1,Size+1}).ToList();_emptySource=ImmutableHashSet<int>.Empty;_listSameElementsWithDuplicates=elements.Concat(elements).ToList();}
#region Fast Path: Same Type and Comparer (Optimized)
[Benchmark(Description="ImmutableHashSet (Proper Subset - O(N))")]publicboolCase_ImmutableHashSet_Proper()=>_sourceSet.IsProperSubsetOf(_immutableLarger);[Benchmark(Description="BCL HashSet (Proper Subset - O(N))")]publicboolCase_BclHashSet_Proper()=>_sourceSet.IsProperSubsetOf(_bclHashSetLarger);
#endregion
#region Early Exit: Count Check (O(1))
[Benchmark(Description="Empty Source (O(1) Check)")]publicboolCase_EmptySource_Proper()=>_emptySource.IsProperSubsetOf(_bclHashSetLarger);[Benchmark(Description="List (Same Elements with Duplicates - Not Proper)")]publicboolCase_List_Duplicates_NotProper()=>_sourceSet.IsProperSubsetOf(_listSameElementsWithDuplicates);[Benchmark(Description="Early Exit (Other is Smaller)")]publicboolCase_SmallerCount()=>_sourceSet.IsProperSubsetOf(_immutableSmaller);[Benchmark(Description="Early Exit (Same Count - Cannot be Proper)")]publicboolCase_SameCount()=>_sourceSet.IsProperSubsetOf(_immutableSameCount);
#endregion
#region Fallback Path: Non-Set or Different Comparer
[Benchmark(Description="List (Proper - Fallback to HashSet)")]publicboolCase_List_Proper()=>_sourceSet.IsProperSubsetOf(_listLarger);[Benchmark(Description="Array (Proper - Fallback to HashSet)")]publicboolCase_Array_Proper()=>_sourceSet.IsProperSubsetOf(_arrayLarger);[Benchmark(Description="HashSet (Different Comparer - Force Fallback)")]publicboolCase_HashSet_DiffComparer()=>_sourceSet.IsProperSubsetOf(_bclHashSetLargerDiffComparer);[Benchmark(Description="List with Duplicates (Proper Subset)")]publicboolCase_List_Duplicates_Proper()=>_sourceSet.IsProperSubsetOf(_listWithDuplicatesButProper);
#endregion
}publicclassReverseComparer<T>:IEqualityComparer<T>whereT:IComparable<T>{publicboolEquals(T?x,T?y)=>x?.CompareTo(y)==0;publicintGetHashCode(T?obj)=>obj?.GetHashCode()??0;}publicclassProgram{publicstaticvoidMain(string[]args){BenchmarkRunner.Run<ImmutableHashSetIsProperSubsetOfBenchmark>();}}}

Click to expand Benchmark Results

Benchmark Results (Before Optimization)

Method	Size	Mean	Error	StdDev	Rank	Gen0	Gen1	Gen2	Allocated
'Empty Source (O(1) Check)'	100000	3.020 ns	0.0522 ns	0.0463 ns	1	-	-	-	-
'List (Same Elements with Duplicates - Not Proper)'	100000	2,253,302.064 ns	43,996.1710 ns	83,707.2944 ns	2	85.9375	85.9375	85.9375	3605725 B
'BCL HashSet (Proper Subset - O(N))'	100000	7,143,590.897 ns	55,965.9984 ns	52,350.6297 ns	3	62.5000	62.5000	62.5000	1738869 B
'Array (Proper - Fallback to HashSet)'	100000	7,191,053.721 ns	60,956.8792 ns	54,036.6857 ns	3	70.3125	70.3125	70.3125	1738731 B
'Early Exit (Other is Smaller)'	100000	7,252,623.997 ns	127,108.7726 ns	112,678.6161 ns	3	70.3125	70.3125	70.3125	1738868 B
'Early Exit (Same Count - Cannot be Proper)'	100000	7,254,561.478 ns	73,899.0961 ns	57,695.5534 ns	3	78.1250	78.1250	78.1250	1738874 B
'List (Proper - Fallback to HashSet)'	100000	8,628,967.714 ns	103,114.2276 ns	96,453.1125 ns	4	78.1250	78.1250	78.1250	1738861 B
'List with Duplicates (Proper Subset)'	100000	8,979,029.530 ns	124,859.7911 ns	104,263.5805 ns	4	78.1250	78.1250	78.1250	1738861 B
'HashSet (Different Comparer - Force Fallback)'	100000	9,236,192.453 ns	94,804.2142 ns	88,679.9208 ns	4	78.1250	78.1250	78.1250	1738861 B
'ImmutableHashSet (Proper Subset - O(N))'	100000	14,980,671.116 ns	155,763.4001 ns	145,701.1812 ns	5	78.1250	78.1250	78.1250	1738897 B

---

Benchmark Results (After Optimization)

Method	Size	Mean	Error	StdDev	Rank	Gen0	Gen1	Gen2	Allocated
'Empty Source (O(1) Check)'	100000	1.769 ns	0.0372 ns	0.0348 ns	1	-	-	-	-
'Early Exit (Same Count - Cannot be Proper)'	100000	2.261 ns	0.0409 ns	0.0363 ns	2	-	-	-	-
'Early Exit (Other is Smaller)'	100000	2.401 ns	0.0800 ns	0.0748 ns	2	-	-	-	-
'List (Same Elements with Duplicates - Not Proper)'	100000	2,226,632.494 ns	44,488.2292 ns	94,808.0507 ns	3	82.0313	82.0313	82.0313	3605636 B
'Array (Proper - Fallback to HashSet)'	100000	4,128,214.156 ns	40,633.6128 ns	36,020.6393 ns	4	62.5000	62.5000	62.5000	1738710 B
'HashSet (Different Comparer - Force Fallback)'	100000	4,310,472.716 ns	39,782.7924 ns	33,220.4335 ns	5	70.3125	70.3125	70.3125	1738810 B
'BCL HashSet (Proper Subset - O(N))'	100000	5,622,467.743 ns	43,428.7884 ns	36,265.0052 ns	6	-	-	-	-
'List (Proper - Fallback to HashSet)'	100000	6,933,851.438 ns	51,688.7203 ns	43,162.4224 ns	7	62.5000	62.5000	62.5000	1738734 B
'List with Duplicates (Proper Subset)'	100000	7,455,423.480 ns	92,394.8283 ns	81,905.6087 ns	8	70.3125	70.3125	70.3125	1738817 B
'ImmutableHashSet (Proper Subset - O(N))'	100000	13,207,980.537 ns	130,270.0225 ns	115,480.9819 ns	9	-	-	-	-

Performance Analysis Summary (100,000 Elements)

Case / Method	Before (ns)	After (ns)	Speedup Ratio	Memory Improvement
Early Exit (Other is Smaller)	7,252,623	2.401	~3,020,667x	-100% (Zero Alloc)
Early Exit (Same Count)	7,254,561	2.261	~3,208,563x	-100% (Zero Alloc)
Empty Source	3.020	1.769	1.71x	Zero Alloc
BCL HashSet (Proper Subset)	7,143,590	5,622,467	1.27x	-100% (Zero Alloc)
List (Duplicates - Not Proper)	2,253,302	2,226,632	1.01x	Stable (3.6 MB)
Array (Fallback to HashSet)	7,191,053	4,128,214	1.74x	Stable (1.7 MB)
List (Proper - Fallback)	8,628,967	6,933,851	1.24x	Stable (1.7 MB)
List with Duplicates (Proper)	8,979,029	7,455,423	1.20x	Stable (1.7 MB)
HashSet (Diff Comparer)	9,236,192	4,310,472	2.14x	Stable (1.7 MB)
ImmutableHashSet (Proper)	14,980,671	13,207,980	1.13x	-100% (Zero Alloc)

---

✅ Unit Tests Added

Added unit tests for IsProperSubsetOf to cover various edge cases and ensure the correctness of the new logic:

• Mismatched Comparers: Validated behavior when comparing sets with different comparers (e.g., Ordinal vs. OrdinalIgnoreCase).
• Duplicate Elements: Verified ICollection<T> logic to ensure that collections with duplicates are handled correctly by the early exit (Count <= origin.Count).
• Empty Set Scenarios: Confirmed expected behavior when either the origin or the target collection is empty.
• Equality Logic: Covered cases where logical equality might differ from reference equality in comparers.

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[aw0lid]

Hi everyone, just a gentle follow-up on this PR
CC/ @eiriktsarpalis, @MihaZupan

[github-actions]

Workflow state for the Holistic Review Orchestrator.

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[github-actions]

Holistic Review

Motivation: ImmutableHashSet<T>.IsProperSubsetOf always materialized a new HashSet<T> from other, incurring an O(n) allocation even in cases that could be resolved in O(1) (e.g. other has fewer or equal elements) or without allocation (e.g. other is already an ImmutableHashSet<T>/HashSet<T> with a compatible comparer). This adds avoidable GC pressure for large sets. This PR is part of #127279.

Approach: The public IsProperSubsetOf now short-circuits the empty-origin case (return other.Any()) up front. The private IsProperSubsetOf(other, origin) gains a switch (other) that mirrors the existing SetEquals fast-path structure: for ImmutableHashSet<T>, HashSet<T>, and ICollection<T> it performs an O(1) count check (Count <= origin.Count => false, correct because a proper subset requires the superset to be strictly larger, and this holds even for collections with duplicates since dedup only shrinks the count). When the comparer matches, it reuses SetEqualsWithImmutableHashset/SetEqualsWithHashset to verify containment without allocating. Only the general IEnumerable fallback still allocates a HashSet<T>. Tests cover mismatched comparers, duplicate ICollection elements, and empty-set scenarios.

Summary: The refactor is correct and consistent with the established SetEquals fast-path pattern in this file. The count checks are logically sound: because origin is a set, a proper-subset relationship requires other's distinct count to strictly exceed origin.Count, and Count <= origin.Count on the raw collection safely rejects (dedup can only reduce the count). The comparer guard via EqualityComparer<IEqualityComparer<T>>.Default.Equals correctly restricts the zero-alloc containment path to compatible comparers; mismatched-comparer cases fall through to the allocating new HashSet<T>(other, origin.EqualityComparer) path, preserving prior semantics. The empty-origin handling moved to the public entry point is equivalent, and the private method's fallback still behaves correctly for empty origin when reached via the Builder (Count <= 0 count checks and the subsequent containment/count logic). I found no correctness regressions. The only issue is a cosmetic indentation glitch on the switch (other) line (flagged inline), which formatting validation may reject. Verdict: LGTM once the formatting nit is addressed.

Detailed Findings

No functional issues found. One cosmetic finding is noted inline: the switch (other) statement at line 940 is mis-indented; run dotnet format to align it with the enclosing block so the C# formatting check passes.

Note

This review was generated by this repository's Holistic Review agentic workflow to complement the built-in Copilot review.

Generated by Holistic Review · 94.9 AIC · ⌖ 10.6 AIC · ⊞ 10K

[github-actions]

Holistic Review

Motivation: ImmutableHashSet<T>.IsProperSubsetOf always materialized a new HashSet<T> from other, incurring an O(n) allocation even in cases resolvable in O(1) (e.g. other has fewer or equal elements) or without allocation (e.g. other is already an ImmutableHashSet<T>/HashSet<T> with a compatible comparer). This adds avoidable GC pressure for large sets. This PR is part of #127279.

Approach: The public IsProperSubsetOf short-circuits the empty-origin case (return other.Any()) up front. The private IsProperSubsetOf(other, origin) gains a switch (other) mirroring the existing SetEquals fast-path structure: for ImmutableHashSet<T>, HashSet<T>, and ICollection<T> it performs an O(1) count check (Count <= origin.Count => false, correct because a proper subset requires the superset to be strictly larger, and this holds even for collections with duplicates since dedup only shrinks the count). When the comparer matches, it reuses SetEqualsWithImmutableHashset/SetEqualsWithHashset to verify containment without allocating. Only the general IEnumerable fallback still allocates a HashSet<T>. Tests cover mismatched comparers, duplicate ICollection elements, and empty-set scenarios.

Summary: The latest commit makes a single change: it corrects the mis-indentation of the switch (other) statement that was flagged in the prior review. It is now aligned (12 spaces) with the enclosing block, so the C# formatting check should pass. No functional code changed in this increment, and the cumulative assessment is unchanged: the refactor is correct, consistent with the established SetEquals fast-path pattern, and introduces no correctness regressions. The count checks are sound, the comparer guard correctly restricts the zero-alloc containment path to compatible comparers, and mismatched-comparer cases fall through to the allocating path preserving prior semantics. The empty-origin handling moved to the public entry point is equivalent. Verdict: LGTM.

Detailed Findings

No functional issues found. The only outstanding item from the prior review—the mis-indented switch (other) line—has been fixed in this increment.

Assessment History

• review 4730525611 reviewed commit e76ce57: verdict LGTM (once the formatting nit was addressed). Current verdict: LGTM. Assessment changed only in that the previously-flagged cosmetic indentation issue is now resolved by commit d5da427; motivation, approach, and risk are otherwise unchanged.

Note

This review was generated by this repository's Holistic Review agentic workflow to complement the built-in Copilot review.

Generated by Holistic Review · 44.7 AIC · ⌖ 9.67 AIC · ⊞ 10K

[aw0lid]

Friendly ping @dotnet/area-system-collections - Re-requesting review on this PR. All CI checks are green.
Could someone from the team take a look when you have a moment? Thanks!