From 41d7ee2c6e91e6ea024d161fc9a36b9b9b1095fe Mon Sep 17 00:00:00 2001
From: Tanner Gooding <tagoo@microsoft.com>
Date: Sat, 11 Jul 2026 11:42:11 -0700
Subject: [PATCH] Simplify StringBuilder indexer to reuse FindChunkForIndex

The get/set indexers open-coded a chunk walk that did two comparisons per
hop and traversed the entire chunk list to validate an out-of-range index.

Hoist a single `(uint)index >= (uint)Length` bounds check up front so the
out-of-range path is O(1) and each hop of the walk is a single comparison,
then reuse the existing `FindChunkForIndex` helper for the traversal. Route
the throws through `ThrowHelper` so the property bodies stay small enough to
inline.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
---
 .../src/System/Text/StringBuilder.cs          | 43 +++++--------------
 1 file changed, 10 insertions(+), 33 deletions(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs b/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs
index 5787996513129a..305795f7d2504f 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs
@@ -487,46 +487,23 @@ public char this[int index]
         {
             get
             {
-                StringBuilder? chunk = this;
-                while (true)
+                if ((uint)index >= (uint)Length)
                 {
-                    int indexInBlock = index - chunk.m_ChunkOffset;
-                    if (indexInBlock >= 0)
-                    {
-                        if (indexInBlock >= chunk.m_ChunkLength)
-                        {
-                            throw new IndexOutOfRangeException();
-                        }
-                        return chunk.m_ChunkChars[indexInBlock];
-                    }
-                    chunk = chunk.m_ChunkPrevious;
-                    if (chunk == null)
-                    {
-                        throw new IndexOutOfRangeException();
-                    }
+                    ThrowHelper.ThrowIndexOutOfRangeException();
                 }
+
+                StringBuilder chunk = FindChunkForIndex(index);
+                return chunk.m_ChunkChars[index - chunk.m_ChunkOffset];
             }
             set
             {
-                StringBuilder? chunk = this;
-                while (true)
+                if ((uint)index >= (uint)Length)
                 {
-                    int indexInBlock = index - chunk.m_ChunkOffset;
-                    if (indexInBlock >= 0)
-                    {
-                        if (indexInBlock >= chunk.m_ChunkLength)
-                        {
-                            throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_IndexMustBeLess);
-                        }
-                        chunk.m_ChunkChars[indexInBlock] = value;
-                        return;
-                    }
-                    chunk = chunk.m_ChunkPrevious;
-                    if (chunk == null)
-                    {
-                        throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_IndexMustBeLess);
-                    }
+                    ThrowHelper.ThrowArgumentOutOfRange_IndexMustBeLessException();
                 }
+
+                StringBuilder chunk = FindChunkForIndex(index);
+                chunk.m_ChunkChars[index - chunk.m_ChunkOffset] = value;
             }
         }