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Pre-size intermediate Dictionary in ToFrozenDictionary #128300

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Merged
MihaZupan merged 6 commits into from
May 19, 2026
+2 −1
Merged

Pre-size intermediate Dictionary in ToFrozenDictionary #128300

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cbeb324
Pre-size intermediate Dictionary in ToFrozenDictionary
AndrewP-GH May 17, 2026
04e65fe
Narrow capacity hint to dictionary-shaped sources
AndrewP-GH May 17, 2026
f6c1460
Potential fix for pull request finding
AndrewP-GH May 17, 2026
a1903df
Drop redundant dictionary arms from capacity switch
AndrewP-GH May 17, 2026
f897544
Merge branch 'main' into perf/frozen-dict-presize
AndrewP-GH May 17, 2026
36c8bcb
Update src/libraries/System.Collections.Immutable/src/System/Collecti…
AndrewP-GH May 19, 2026
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10 changes: 9 additions & 1 deletion src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -131,7 +131,15 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
newDictionary = source as Dictionary<TKey, TValue>;
if (newDictionary is null || (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer)))
{
newDictionary = new Dictionary<TKey, TValue>(comparer);
int capacity = source switch
{
IDictionary<TKey, TValue> dictionary => dictionary.Count,
IReadOnlyDictionary<TKey, TValue> readOnlyDictionary => readOnlyDictionary.Count,
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ICollection<KeyValuePair<TKey, TValue>> collection => collection.Count,
IReadOnlyCollection<KeyValuePair<TKey, TValue>> readOnlyCollection => readOnlyCollection.Count,
_ => 0,
};
newDictionary = new Dictionary<TKey, TValue>(capacity, comparer);
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foreach (KeyValuePair<TKey, TValue> pair in source)
{
// Dictionary's constructor uses Add, which will throw on duplicates.
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