From 5b26d190a6912f7a0427b90f738a6c51351ec1f6 Mon Sep 17 00:00:00 2001
From: prozolic <42107886+prozolic@users.noreply.github.com>
Date: Thu, 16 Oct 2025 20:58:46 +0900
Subject: [PATCH 1/3] Improve FrozenDictionary.ToFrozenDictionary for
 Dictionary sources with different comparers

---
 .../Collections/Frozen/FrozenDictionary.cs       | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

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
index b9ddcb1b4e484f..c2a3aaa236f080 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
@@ -129,7 +129,7 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
             // Ensure we have a Dictionary<,> using the specified comparer such that all keys
             // are non-null and unique according to that comparer.
             newDictionary = source as Dictionary<TKey, TValue>;
-            if (newDictionary is null || (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer)))
+            if (newDictionary is null)
             {
                 newDictionary = new Dictionary<TKey, TValue>(comparer);
                 foreach (KeyValuePair<TKey, TValue> pair in source)
@@ -140,6 +140,18 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
                     newDictionary[pair.Key] = pair.Value;
                 }
             }
+            else if (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer))
+            {
+                var dictionary = new Dictionary<TKey, TValue>(newDictionary.Count, comparer);
+                foreach (KeyValuePair<TKey, TValue> pair in newDictionary)
+                {
+                    // Dictionary's constructor uses Add, which will throw on duplicates.
+                    // This implementation uses the indexer to avoid throwing and to overwrite
+                    // existing entries such that last one wins.
+                    dictionary[pair.Key] = pair.Value;
+                }
+                newDictionary = dictionary;
+            }
 
             if (newDictionary.Count == 0)
             {

From 9fff89d0f272439fd746299bdaa07c36c1bc74c5 Mon Sep 17 00:00:00 2001
From: prozolic <42107886+prozolic@users.noreply.github.com>
Date: Thu, 16 Oct 2025 21:33:21 +0900
Subject: [PATCH 2/3] Fix file encoding to UTF-8 with BOM

---
 .../src/System/Collections/Frozen/FrozenDictionary.cs           | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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
index c2a3aaa236f080..90f5e5a019eab6 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;

From 089e68bd92668192cbebfeb9137cf80e4e6ee21f Mon Sep 17 00:00:00 2001
From: prozolic <42107886+prozolic@users.noreply.github.com>
Date: Thu, 16 Oct 2025 22:48:04 +0900
Subject: [PATCH 3/3] Remove initial capacity specification

---
 .../src/System/Collections/Frozen/FrozenDictionary.cs           | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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
index 90f5e5a019eab6..1fbfba41561add 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs
@@ -142,7 +142,7 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
             }
             else if (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer))
             {
-                var dictionary = new Dictionary<TKey, TValue>(newDictionary.Count, comparer);
+                var dictionary = new Dictionary<TKey, TValue>(comparer);
                 foreach (KeyValuePair<TKey, TValue> pair in newDictionary)
                 {
                     // Dictionary's constructor uses Add, which will throw on duplicates.