Pre-size intermediate Dictionary in ToFrozenDictionary#128300
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AndrewP-GH wants to merge 5 commits into
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Pre-size intermediate Dictionary in ToFrozenDictionary#128300AndrewP-GH wants to merge 5 commits into
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Pull request overview
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Improves ToFrozenDictionary construction performance by pre-sizing the intermediate Dictionary when the source reports a count, and adds a regression test to ensure duplicate-key “last in wins” semantics remain intact for ICollection sources.
Changes:
- Pre-size the intermediate
Dictionary<TKey, TValue>using the source collection’s count when available. - Add a unit test covering duplicate keys for
ICollectionsources to prevent regressions in overwrite semantics.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs | Pre-sizes intermediate dictionary based on source count to reduce resizes/rehashes during population. |
| src/libraries/System.Collections.Immutable/tests/Frozen/FrozenDictionaryTests.cs | Adds regression coverage to ensure pre-sizing doesn’t change duplicate-key overwrite behavior. |
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When the source is not already a Dictionary, ToFrozenDictionary builds an intermediate Dictionary to deduplicate keys with last-wins semantics. The previous code used the default-capacity constructor, which paid log2(N) resizes and ~2N redundant rehashes when populating from a known size source such as an array or List. Use ICollection<KeyValuePair>.Count as a capacity hint when available, mirroring the HashSet(IEnumerable, IEqualityComparer) constructor. The ReadOnlySpan overload of FrozenDictionary.Create already pre-sizes, so this brings the IEnumerable path in line with it.
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Switch the source-type check from ICollection<KeyValuePair> to IDictionary<,> / IReadOnlyDictionary<,>. For dictionary inputs Count reflects already-unique keys, so the intermediate Dictionary is sized exactly without risk of over-allocation from duplicate-bearing collections such as KeyValuePair[].
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Co-authored-by: Copilot Autofix powered by AI <[email protected]>
IDictionary<TKey, TValue> extends ICollection<KeyValuePair<TKey, TValue>> and IReadOnlyDictionary<TKey, TValue> extends IReadOnlyCollection<KeyValuePair<TKey, TValue>>, and Count is inherited from the collection interfaces. The dictionary arms always matched the collection arms with the same value, so they were dead code. Matches the pattern Dictionary<,> and ConcurrentDictionary<,> use in their own IEnumerable constructors.
This was referenced May 17, 2026
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ToFrozenDictionarybuilds an intermediateDictionary<,>to deduplicate keys when the source isn't already a matchingDictionary<,>. It used the default-capacity constructor, so a known-size source payslog2(N)resizes and ~2Nrehashes for nothing.Helps most where the early
source as Dictionary<,>reuse path doesn't fire:ConcurrentDictionary<,>,SortedDictionary<,>,ImmutableDictionary<,>,ReadOnlyDictionary<,>, plus arrays andList<KVP>from LINQ.Sizing-only change —
Dictionary<,>capacity is contractually behavior-invariant, so existingMultipleValuesSameKey_LastInSourceWinsalready covers the sharedforeach { d[k] = v; }body.