From 88633ce738420340f2839d9b87f44f346b638fe4 Mon Sep 17 00:00:00 2001
From: prozolic <42107886+prozolic@users.noreply.github.com>
Date: Wed, 8 Jul 2026 22:10:25 +0900
Subject: [PATCH 1/2] Add fast path in string.Concat

This PR adds a fast path for string.Concat when the input value is
List<string?> or string?[]. This fast path is based on the pattern
already used in Join(string? separator, IEnumerable<string?>
values).
I have also added tests for the fast path and fallback path.
---
 src/libraries/Common/tests/Tests/System/StringTests.cs |  2 ++
 .../src/System/String.Manipulation.cs                  | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/src/libraries/Common/tests/Tests/System/StringTests.cs b/src/libraries/Common/tests/Tests/System/StringTests.cs
index 806c39efa529a6..64ed05a6a699b9 100644
--- a/src/libraries/Common/tests/Tests/System/StringTests.cs
+++ b/src/libraries/Common/tests/Tests/System/StringTests.cs
@@ -317,6 +317,8 @@ void Validate(string result)
             Validate(string.Concat((ReadOnlySpan<string?>)values));
             Validate(string.Concat((IEnumerable<string>)values));
             Validate(string.Concat<string>((IEnumerable<string>)values)); // Call the generic IEnumerable<T>-based overload
+            Validate(string.Concat(values.Select(s => s)));
+            Validate(string.Concat(new List<string?>(values)));
         }
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
index 459e2afc50b7ba..367336198c4dc6 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
@@ -152,6 +152,16 @@ public static unsafe string Concat(IEnumerable<string?> values)
         {
             ArgumentNullException.ThrowIfNull(values);
 
+            if (values.GetType() == typeof(List<string?>))
+            {
+                return Concat(CollectionsMarshal.AsSpan((List<string?>)values));
+            }
+
+            if (values is string?[] valuesArray)
+            {
+                return Concat((ReadOnlySpan<string?>)valuesArray);
+            }
+
             using (IEnumerator<string?> en = values.GetEnumerator())
             {
                 if (!en.MoveNext())

From fbe8720dc59fccbf1b24b092931349d7c3c64419 Mon Sep 17 00:00:00 2001
From: prozolic <42107886+prozolic@users.noreply.github.com>
Date: Thu, 9 Jul 2026 01:57:12 +0900
Subject: [PATCH 2/2] Update
 src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

Co-authored-by: Miha Zupan <mihazupan.zupan1@gmail.com>
---
 .../System.Private.CoreLib/src/System/String.Manipulation.cs    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
index 367336198c4dc6..a2999577c2531c 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
@@ -152,7 +152,7 @@ public static unsafe string Concat(IEnumerable<string?> values)
         {
             ArgumentNullException.ThrowIfNull(values);
 
-            if (values.GetType() == typeof(List<string?>))
+            if (values.GetType() == typeof(List<string?>)) // avoid accidentally bypassing a derived type's reimplementation of IEnumerable<T>
             {
                 return Concat(CollectionsMarshal.AsSpan((List<string?>)values));
             }