Pre-size intermediate Dictionary in ToFrozenDictionary (#128300)

AndrewP-GH · Copilot · MihaZupan · web-flow · commit 14c1d484890b · 2026-05-19T22:27:59.000Z
Co-authored-by: Copilot Autofix powered by AI &lt;175728472+Copilot@users.noreply.github.com&gt;
Co-authored-by: Miha Zupan &lt;mihazupan.zupan1@gmail.com&gt;
diff --git a/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs b/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
@@ -131,7 +131,8 @@ 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 as ICollection<KeyValuePair<TKey, TValue>>)?.Count ?? 0;
+                newDictionary = new Dictionary<TKey, TValue>(capacity, comparer);
                 foreach (KeyValuePair<TKey, TValue> pair in source)
                 {
                     // Dictionary's constructor uses Add, which will throw on duplicates.

Original file line number	Diff line number	Diff line change
`@@ -131,7 +131,8 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,`
`131`	`131`	`newDictionary = source as Dictionary<TKey, TValue>;`
`132`	`132`	`if (newDictionary is null \|\| (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer)))`
`133`	`133`	`{`
`134`		`- newDictionary = new Dictionary<TKey, TValue>(comparer);`
	`134`	`+ int capacity = (source as ICollection<KeyValuePair<TKey, TValue>>)?.Count ?? 0;`
	`135`	`+ newDictionary = new Dictionary<TKey, TValue>(capacity, comparer);`
`135`	`136`	`foreach (KeyValuePair<TKey, TValue> pair in source)`
`136`	`137`	`{`
`137`	`138`	`// Dictionary's constructor uses Add, which will throw on duplicates.`