From 9d5a379e109e2e1ef0b875a6d7e6aa0d868834b1 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 01:16:22 +0000
Subject: [PATCH 1/7] Initial plan


From e9617793d16169cc1621692a30ce3b387229ab9f Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 01:27:24 +0000
Subject: [PATCH 2/7] Use a pooled StringBuilder for all instances

Agent-Logs-Url: https://github.com/dotnet/Open-XML-SDK/sessions/64d34d63-d66c-4446-b6e3-3697d0681811

Co-authored-by: twsouthwick <583206+twsouthwick@users.noreply.github.com>
---
 .../OpenXmlCompositeElement.cs                |  5 +-
 .../SimpleTypes/HexStringFactory.cs           |  8 +--
 .../SimpleTypes/ListValue.cs                  |  5 +-
 .../System/StringBuilderPool.cs               | 71 +++++++++++++++++++
 .../AttributeAbsentConditionToNonValue.cs     |  5 +-
 .../AttributeAbsentConditionToValue.cs        |  5 +-
 .../Semantic/AttributeMutualExclusive.cs      | 11 +--
 .../AttributeValueConditionToAnother.cs       |  9 ++-
 .../XmlPath.cs                                |  5 +-
 9 files changed, 93 insertions(+), 31 deletions(-)
 create mode 100644 src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs

diff --git a/src/DocumentFormat.OpenXml.Framework/OpenXmlCompositeElement.cs b/src/DocumentFormat.OpenXml.Framework/OpenXmlCompositeElement.cs
index 9a7312433..8b76fe681 100644
--- a/src/DocumentFormat.OpenXml.Framework/OpenXmlCompositeElement.cs
+++ b/src/DocumentFormat.OpenXml.Framework/OpenXmlCompositeElement.cs
@@ -8,7 +8,6 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.IO;
-using System.Text;
 using System.Xml;
 
 namespace DocumentFormat.OpenXml
@@ -122,14 +121,14 @@ public override string InnerText
         {
             get
             {
-                var innerText = new StringBuilder();
+                var innerText = StringBuilderPool.Acquire();
 
                 foreach (var child in ChildElements)
                 {
                     innerText.Append(child.InnerText);
                 }
 
-                return innerText.ToString();
+                return StringBuilderPool.GetValueAndRelease(innerText);
             }
         }
 
diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
index 196a432ee..796eaef83 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
@@ -3,10 +3,6 @@
 
 using System;
 
-#if !NET6_0_OR_GREATER
-using System.Text;
-#endif
-
 namespace DocumentFormat.OpenXml
 {
     /// <summary>
@@ -43,7 +39,7 @@ static string Create(ReadOnlySpan<byte> bytes)
 
             return new string(chars);
 #else
-            var sb = new StringBuilder(bytes.Length * 2);
+            var sb = StringBuilderPool.Acquire();
 
             foreach (var b in bytes)
             {
@@ -51,7 +47,7 @@ static string Create(ReadOnlySpan<byte> bytes)
                 sb.Append(ToCharUpper(b));
             }
 
-            return sb.ToString();
+            return StringBuilderPool.GetValueAndRelease(sb);
 #endif
 
             static char ToCharUpper(int value)
diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs
index c4a1b2cad..782014aa4 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs
@@ -7,7 +7,6 @@
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Diagnostics;
-using System.Text;
 
 namespace DocumentFormat.OpenXml
 {
@@ -194,7 +193,7 @@ public override string? InnerText
             {
                 if (TextValue is null && _list is not null)
                 {
-                    var textString = new StringBuilder();
+                    var textString = StringBuilderPool.Acquire();
                     string separator = string.Empty;
 
                     foreach (var value in _list)
@@ -207,7 +206,7 @@ public override string? InnerText
                         }
                     }
 
-                    TextValue = textString.ToString();
+                    TextValue = StringBuilderPool.GetValueAndRelease(textString);
                 }
 
                 return TextValue;
diff --git a/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs b/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
new file mode 100644
index 000000000..4b11e241c
--- /dev/null
+++ b/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
@@ -0,0 +1,71 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Text;
+
+namespace DocumentFormat.OpenXml;
+
+/// <summary>
+/// Provides a reusable pool of <see cref="StringBuilder"/> instances to reduce allocations.
+/// </summary>
+internal static class StringBuilderPool
+{
+    // Two slots per thread to handle cases where two StringBuilders are active simultaneously.
+    [System.ThreadStatic]
+    private static StringBuilder? _primary;
+
+    [System.ThreadStatic]
+    private static StringBuilder? _secondary;
+
+    /// <summary>
+    /// Acquires a <see cref="StringBuilder"/> from the pool, or creates a new one if the pool is empty.
+    /// </summary>
+    public static StringBuilder Acquire()
+    {
+        var sb = _primary;
+        if (sb is not null)
+        {
+            _primary = null;
+            return sb;
+        }
+
+        sb = _secondary;
+        if (sb is not null)
+        {
+            _secondary = null;
+            return sb;
+        }
+
+        return new StringBuilder();
+    }
+
+    /// <summary>
+    /// Returns the string value of the <see cref="StringBuilder"/> and releases it back to the pool.
+    /// </summary>
+    public static string GetValueAndRelease(StringBuilder sb)
+    {
+        var result = sb.ToString();
+        Release(sb);
+        return result;
+    }
+
+    /// <summary>
+    /// Releases a <see cref="StringBuilder"/> back to the pool after clearing it.
+    /// </summary>
+    public static void Release(StringBuilder sb)
+    {
+        // Use Length = 0 instead of Clear() for .NET 3.5 compatibility
+        sb.Length = 0;
+
+        if (_primary is null)
+        {
+            _primary = sb;
+        }
+        else if (_secondary is null)
+        {
+            _secondary = sb;
+        }
+
+        // If both slots are full, the instance is abandoned and will be garbage collected.
+    }
+}
diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs
index 74bf90d85..4a6a5b036 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs
@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using DocumentFormat.OpenXml.Framework;
-using System.Text;
 
 namespace DocumentFormat.OpenXml.Validation.Semantic
 {
@@ -60,7 +59,7 @@ public AttributeAbsentConditionToNonValue(OpenXmlQualifiedName absentAttribute,
                 }
             }
 
-            var sb = new StringBuilder();
+            var sb = StringBuilderPool.Acquire();
             sb.Append('\'').Append(_values[0]).Append('\'');
             if (_values.Length > 1)
             {
@@ -72,7 +71,7 @@ public AttributeAbsentConditionToNonValue(OpenXmlQualifiedName absentAttribute,
                 sb.Append(" and '").Append(_values[_values.Length - 1]).Append('\'');
             }
 
-            string valueString = sb.ToString();
+            string valueString = StringBuilderPool.GetValueAndRelease(sb);
 
             return new ValidationErrorInfo()
             {
diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs
index 0bd5f3c0d..6de68d1a7 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs
@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using DocumentFormat.OpenXml.Framework;
-using System.Text;
 
 namespace DocumentFormat.OpenXml.Validation.Semantic
 {
@@ -56,7 +55,7 @@ public AttributeAbsentConditionToValue(OpenXmlQualifiedName absentAttribute, Ope
             {
                 if (AttributeValueEquals(conditionAttribute.Value, value, false))
                 {
-                    var sb = new StringBuilder();
+                    var sb = StringBuilderPool.Acquire();
                     sb.Append('\'').Append(_values[0]).Append('\'');
                     if (_values.Length > 1)
                     {
@@ -68,7 +67,7 @@ public AttributeAbsentConditionToValue(OpenXmlQualifiedName absentAttribute, Ope
                         sb.Append(" or '").Append(_values[_values.Length - 1]).Append('\'');
                     }
 
-                    string valueString = sb.ToString();
+                    string valueString = StringBuilderPool.GetValueAndRelease(sb);
 
                     return new ValidationErrorInfo()
                     {
diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeMutualExclusive.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeMutualExclusive.cs
index 8ad401a7c..d64d8e9fd 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeMutualExclusive.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeMutualExclusive.cs
@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using DocumentFormat.OpenXml.Framework;
-using System.Text;
 
 namespace DocumentFormat.OpenXml.Validation.Semantic
 {
@@ -58,8 +57,8 @@ private AttributeMutualExclusive(OpenXmlQualifiedName[] attributes)
                 return null;
             }
 
-            var attributesSb = new StringBuilder();
-            var existAttributeSb = new StringBuilder();
+            var attributesSb = StringBuilderPool.Acquire();
+            var existAttributeSb = StringBuilderPool.Acquire();
             string? existAttribute2 = null;
 
             foreach (var attribute in _attributes)
@@ -94,6 +93,8 @@ private AttributeMutualExclusive(OpenXmlQualifiedName[] attributes)
 
             if (existAttributeSb.Length == 0)
             {
+                StringBuilderPool.Release(attributesSb);
+                StringBuilderPool.Release(existAttributeSb);
                 return null;
             }
 
@@ -104,9 +105,9 @@ private AttributeMutualExclusive(OpenXmlQualifiedName[] attributes)
                 Node = element,
                 Description = SR.Format(
                     ValidationResources.Sem_AttributeMutualExclusive,
-                    existAttributeSb.ToString(),
+                    StringBuilderPool.GetValueAndRelease(existAttributeSb),
                     existAttribute2,
-                    attributesSb.ToString()),
+                    StringBuilderPool.GetValueAndRelease(attributesSb)),
             };
         }
     }
diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs
index 37744f78f..a699dcde9 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs
@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using DocumentFormat.OpenXml.Framework;
-using System.Text;
 
 namespace DocumentFormat.OpenXml.Validation.Semantic
 {
@@ -66,7 +65,7 @@ public AttributeValueConditionToAnother(OpenXmlQualifiedName attribute, OpenXmlQ
             {
                 if (AttributeValueEquals(conditionAttribute.Value, value, false))
                 {
-                    var sb = new StringBuilder();
+                    var sb = StringBuilderPool.Acquire();
                     sb.Append('\'').Append(_values[0]).Append('\'');
                     if (_values.Length > 1)
                     {
@@ -78,9 +77,9 @@ public AttributeValueConditionToAnother(OpenXmlQualifiedName attribute, OpenXmlQ
                         sb.Append(" or '").Append(_values[_values.Length - 1]).Append('\'');
                     }
 
-                    string attributeValueString = sb.ToString();
+                    string attributeValueString = StringBuilderPool.GetValueAndRelease(sb);
 
-                    var otherSb = new StringBuilder();
+                    var otherSb = StringBuilderPool.Acquire();
                     otherSb.Append('\'').Append(_otherValues[0]).Append('\'');
                     if (_otherValues.Length > 1)
                     {
@@ -92,7 +91,7 @@ public AttributeValueConditionToAnother(OpenXmlQualifiedName attribute, OpenXmlQ
                         otherSb.Append(" or '").Append(_otherValues[_otherValues.Length - 1]).Append('\'');
                     }
 
-                    string otherAttributeValueString = otherSb.ToString();
+                    string otherAttributeValueString = StringBuilderPool.GetValueAndRelease(otherSb);
 
                     return new ValidationErrorInfo()
                     {
diff --git a/src/DocumentFormat.OpenXml.Framework/XmlPath.cs b/src/DocumentFormat.OpenXml.Framework/XmlPath.cs
index 510c9dfa7..8478b9423 100644
--- a/src/DocumentFormat.OpenXml.Framework/XmlPath.cs
+++ b/src/DocumentFormat.OpenXml.Framework/XmlPath.cs
@@ -9,7 +9,6 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
-using System.Text;
 using System.Xml;
 
 namespace DocumentFormat.OpenXml
@@ -113,7 +112,7 @@ private static string TryBuildXPath(Stack<OpenXmlElement> elements, out XmlNames
                 return string.Empty;
             }
 
-            var xpath = new StringBuilder();
+            var xpath = StringBuilderPool.Acquire();
             namespaces = new XmlNamespaceManager(new NameTable());
 
             foreach (var element in elements)
@@ -152,7 +151,7 @@ private static string TryBuildXPath(Stack<OpenXmlElement> elements, out XmlNames
                 }
             }
 
-            return xpath.ToString();
+            return StringBuilderPool.GetValueAndRelease(xpath);
         }
 
         private static Stack<OpenXmlElement> GetElements(OpenXmlElement? element)

From 31bceaa11bc294e7c07f7ec5384902eba926da22 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 01:32:24 +0000
Subject: [PATCH 3/7] Address code review: add capacity cap and EnsureCapacity
 hint

Agent-Logs-Url: https://github.com/dotnet/Open-XML-SDK/sessions/64d34d63-d66c-4446-b6e3-3697d0681811

Co-authored-by: twsouthwick <583206+twsouthwick@users.noreply.github.com>
---
 .../SimpleTypes/HexStringFactory.cs                      | 1 +
 .../System/StringBuilderPool.cs                          | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
index 796eaef83..9b67da17f 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
@@ -40,6 +40,7 @@ static string Create(ReadOnlySpan<byte> bytes)
             return new string(chars);
 #else
             var sb = StringBuilderPool.Acquire();
+            sb.EnsureCapacity(bytes.Length * 2);
 
             foreach (var b in bytes)
             {
diff --git a/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs b/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
index 4b11e241c..7f62dfe2e 100644
--- a/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
+++ b/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
@@ -10,6 +10,9 @@ namespace DocumentFormat.OpenXml;
 /// </summary>
 internal static class StringBuilderPool
 {
+    // Maximum capacity (in characters) a StringBuilder may have before it is not returned to the pool.
+    private const int MaxBuilderCapacity = 360;
+
     // Two slots per thread to handle cases where two StringBuilders are active simultaneously.
     [System.ThreadStatic]
     private static StringBuilder? _primary;
@@ -54,6 +57,12 @@ public static string GetValueAndRelease(StringBuilder sb)
     /// </summary>
     public static void Release(StringBuilder sb)
     {
+        // Don't pool excessively large instances to avoid holding onto large memory allocations.
+        if (sb.Capacity > MaxBuilderCapacity)
+        {
+            return;
+        }
+
         // Use Length = 0 instead of Clear() for .NET 3.5 compatibility
         sb.Length = 0;
 

From 725c004f7e53803e20943d058ea1877719e45660 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 02:14:13 +0000
Subject: [PATCH 4/7] Replace ThreadStatic pool with shared
 DefaultObjectPool-style pool; move to Framework/; use InternalsVisibleTo for
 Features

Agent-Logs-Url: https://github.com/dotnet/Open-XML-SDK/sessions/b9e3379d-a520-4210-aa32-03e0ec470e6e

Co-authored-by: twsouthwick <583206+twsouthwick@users.noreply.github.com>
---
 .../Framework/StringBuilderPool.cs            | 103 ++++++++++++++++++
 .../Properties/Properties.cs                  |   1 +
 .../SimpleTypes/HexStringFactory.cs           |   3 +-
 .../SimpleTypes/ListValue.cs                  |   1 +
 .../System/StringBuilderPool.cs               |  80 --------------
 5 files changed, 107 insertions(+), 81 deletions(-)
 create mode 100644 src/DocumentFormat.OpenXml.Framework/Framework/StringBuilderPool.cs
 delete mode 100644 src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs

diff --git a/src/DocumentFormat.OpenXml.Framework/Framework/StringBuilderPool.cs b/src/DocumentFormat.OpenXml.Framework/Framework/StringBuilderPool.cs
new file mode 100644
index 000000000..deac3307d
--- /dev/null
+++ b/src/DocumentFormat.OpenXml.Framework/Framework/StringBuilderPool.cs
@@ -0,0 +1,103 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Text;
+using System.Threading;
+
+#if !NET35
+using System;
+using System.Collections.Concurrent;
+#endif
+
+namespace DocumentFormat.OpenXml.Framework
+{
+    /// <summary>
+    /// Provides a shared, reusable pool of <see cref="StringBuilder"/> instances to reduce allocations.
+    /// </summary>
+    /// <remarks>
+    /// Design mirrors DefaultObjectPool from Microsoft.Extensions.ObjectPool:
+    /// a single lock-free fast-path slot plus a bounded overflow queue capped at
+    /// <c>Environment.ProcessorCount * 2</c>.
+    /// </remarks>
+    internal static class StringBuilderPool
+    {
+        // Maximum capacity (in characters) a StringBuilder may have before it is not returned to the pool.
+        private const int MaxBuilderCapacity = 360;
+
+#if !NET35
+        // Secondary queue for concurrent overflow; capacity is ProcessorCount * 2 minus 1 for _fastItem.
+        private static readonly ConcurrentQueue<StringBuilder> _items = new ConcurrentQueue<StringBuilder>();
+        private static readonly int _maxCapacity = (Environment.ProcessorCount * 2) - 1;
+#endif
+
+        // Single fast-path slot; accessed via Interlocked.CompareExchange for lock-free hand-off.
+        private static StringBuilder? _fastItem;
+
+#if !NET35
+        private static int _numItems;
+#endif
+
+        /// <summary>
+        /// Acquires a <see cref="StringBuilder"/> from the pool, or creates a new one if the pool is empty.
+        /// </summary>
+        public static StringBuilder Acquire()
+        {
+            var item = _fastItem;
+            if (item != null && Interlocked.CompareExchange(ref _fastItem, null, item) == item)
+            {
+                return item;
+            }
+
+#if !NET35
+            if (_items.TryDequeue(out item))
+            {
+                Interlocked.Decrement(ref _numItems);
+                return item;
+            }
+#endif
+
+            return new StringBuilder();
+        }
+
+        /// <summary>
+        /// Returns the string value of the <see cref="StringBuilder"/> and releases it back to the pool.
+        /// </summary>
+        public static string GetValueAndRelease(StringBuilder sb)
+        {
+            var result = sb.ToString();
+            Release(sb);
+            return result;
+        }
+
+        /// <summary>
+        /// Releases a <see cref="StringBuilder"/> back to the pool after clearing it.
+        /// </summary>
+        public static void Release(StringBuilder sb)
+        {
+            // Don't pool excessively large instances to avoid retaining large memory allocations.
+            if (sb.Capacity > MaxBuilderCapacity)
+            {
+                return;
+            }
+
+            // Use Length = 0 instead of Clear() for .NET 3.5 compatibility.
+            sb.Length = 0;
+
+            if (Interlocked.CompareExchange(ref _fastItem, sb, null) == null)
+            {
+                return;
+            }
+
+#if !NET35
+            if (Interlocked.Increment(ref _numItems) <= _maxCapacity)
+            {
+                _items.Enqueue(sb);
+                return;
+            }
+
+            // Pool is full; drop the instance so it can be GC'd.
+            Interlocked.Decrement(ref _numItems);
+#endif
+        }
+    }
+}
diff --git a/src/DocumentFormat.OpenXml.Framework/Properties/Properties.cs b/src/DocumentFormat.OpenXml.Framework/Properties/Properties.cs
index 3fd12daf7..3571df3a7 100644
--- a/src/DocumentFormat.OpenXml.Framework/Properties/Properties.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Properties/Properties.cs
@@ -6,6 +6,7 @@
 using System.Runtime.CompilerServices;
 
 [assembly: InternalsVisibleTo("DocumentFormat.OpenXml, Publickey=002400000480000094000000060200000024000052534131000400000100010061d8931836c82bf25ca6b773dfd6e7b3ab4e43fba60cf4a86347170373415a165ccc40da3da4a52163822db9fa91f15828236d32d6a9fe754859f10d1f8262646c1f3fb6b4348123f14d733db0ff11c3198b7cf56caaebbf14563990446a6c32aff36d5a7097194294c127fe3cdf9f2609daae5f4daf26f8b6227f203d2a8bbf")]
+[assembly: InternalsVisibleTo("DocumentFormat.OpenXml.Features, Publickey=002400000480000094000000060200000024000052534131000400000100010061d8931836c82bf25ca6b773dfd6e7b3ab4e43fba60cf4a86347170373415a165ccc40da3da4a52163822db9fa91f15828236d32d6a9fe754859f10d1f8262646c1f3fb6b4348123f14d733db0ff11c3198b7cf56caaebbf14563990446a6c32aff36d5a7097194294c127fe3cdf9f2609daae5f4daf26f8b6227f203d2a8bbf")]
 [assembly: InternalsVisibleTo("DocumentFormat.OpenXml.Linq, Publickey=002400000480000094000000060200000024000052534131000400000100010061d8931836c82bf25ca6b773dfd6e7b3ab4e43fba60cf4a86347170373415a165ccc40da3da4a52163822db9fa91f15828236d32d6a9fe754859f10d1f8262646c1f3fb6b4348123f14d733db0ff11c3198b7cf56caaebbf14563990446a6c32aff36d5a7097194294c127fe3cdf9f2609daae5f4daf26f8b6227f203d2a8bbf")]
 
 // For testing
diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
index 9b67da17f..4bb35a7a1 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using DocumentFormat.OpenXml.Framework;
 using System;
 
 namespace DocumentFormat.OpenXml
@@ -70,6 +71,6 @@ static char ToCharUpper(int value)
         /// </summary>
         /// <param name="bytes">A byte array to use to create a new hex string.</param>
         /// <returns>A hex string that corresponds to the value parameter.</returns>
-        public static string Create(params byte[] bytes) => Create(bytes.AsSpan());
+        public static string Create(params byte[] bytes) => Create(new ReadOnlySpan<byte>(bytes));
     }
 }
diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs
index 782014aa4..78bcb1784 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs
@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections;
 using System.Collections.Generic;
diff --git a/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs b/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
deleted file mode 100644
index 7f62dfe2e..000000000
--- a/src/DocumentFormat.OpenXml.Framework/System/StringBuilderPool.cs
+++ /dev/null
@@ -1,80 +0,0 @@
-// Copyright (c) Microsoft. All rights reserved.
-// Licensed under the MIT license. See LICENSE file in the project root for full license information.
-
-using System.Text;
-
-namespace DocumentFormat.OpenXml;
-
-/// <summary>
-/// Provides a reusable pool of <see cref="StringBuilder"/> instances to reduce allocations.
-/// </summary>
-internal static class StringBuilderPool
-{
-    // Maximum capacity (in characters) a StringBuilder may have before it is not returned to the pool.
-    private const int MaxBuilderCapacity = 360;
-
-    // Two slots per thread to handle cases where two StringBuilders are active simultaneously.
-    [System.ThreadStatic]
-    private static StringBuilder? _primary;
-
-    [System.ThreadStatic]
-    private static StringBuilder? _secondary;
-
-    /// <summary>
-    /// Acquires a <see cref="StringBuilder"/> from the pool, or creates a new one if the pool is empty.
-    /// </summary>
-    public static StringBuilder Acquire()
-    {
-        var sb = _primary;
-        if (sb is not null)
-        {
-            _primary = null;
-            return sb;
-        }
-
-        sb = _secondary;
-        if (sb is not null)
-        {
-            _secondary = null;
-            return sb;
-        }
-
-        return new StringBuilder();
-    }
-
-    /// <summary>
-    /// Returns the string value of the <see cref="StringBuilder"/> and releases it back to the pool.
-    /// </summary>
-    public static string GetValueAndRelease(StringBuilder sb)
-    {
-        var result = sb.ToString();
-        Release(sb);
-        return result;
-    }
-
-    /// <summary>
-    /// Releases a <see cref="StringBuilder"/> back to the pool after clearing it.
-    /// </summary>
-    public static void Release(StringBuilder sb)
-    {
-        // Don't pool excessively large instances to avoid holding onto large memory allocations.
-        if (sb.Capacity > MaxBuilderCapacity)
-        {
-            return;
-        }
-
-        // Use Length = 0 instead of Clear() for .NET 3.5 compatibility
-        sb.Length = 0;
-
-        if (_primary is null)
-        {
-            _primary = sb;
-        }
-        else if (_secondary is null)
-        {
-            _secondary = sb;
-        }
-
-        // If both slots are full, the instance is abandoned and will be garbage collected.
-    }
-}

From 490955e04a852d755c2fff07e7d6504de53bbef3 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 02:14:56 +0000
Subject: [PATCH 5/7] Add comment explaining ReadOnlySpan constructor use in
 HexStringFactory

Agent-Logs-Url: https://github.com/dotnet/Open-XML-SDK/sessions/b9e3379d-a520-4210-aa32-03e0ec470e6e

Co-authored-by: twsouthwick <583206+twsouthwick@users.noreply.github.com>
---
 .../SimpleTypes/HexStringFactory.cs                             | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
index 4bb35a7a1..75a458006 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
@@ -71,6 +71,8 @@ static char ToCharUpper(int value)
         /// </summary>
         /// <param name="bytes">A byte array to use to create a new hex string.</param>
         /// <returns>A hex string that corresponds to the value parameter.</returns>
+        // Use constructor instead of bytes.AsSpan() to avoid SpanExtensions ambiguity
+        // when this file is linked into DocumentFormat.OpenXml.Features.
         public static string Create(params byte[] bytes) => Create(new ReadOnlySpan<byte>(bytes));
     }
 }

From c6244f4c323a6efe29ebef4432d6b53d00c163ca Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 02:25:58 +0000
Subject: [PATCH 6/7] Un-chain .Append calls to one per line for readability

Agent-Logs-Url: https://github.com/dotnet/Open-XML-SDK/sessions/07f56062-8462-44e4-9b69-bba899c6df89

Co-authored-by: twsouthwick <583206+twsouthwick@users.noreply.github.com>
---
 .../AttributeAbsentConditionToNonValue.cs     | 12 +++++++---
 .../AttributeAbsentConditionToValue.cs        | 12 +++++++---
 .../AttributeValueConditionToAnother.cs       | 24 ++++++++++++++-----
 3 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs
index 4a6a5b036..0ae39800f 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToNonValue.cs
@@ -60,15 +60,21 @@ public AttributeAbsentConditionToNonValue(OpenXmlQualifiedName absentAttribute,
             }
 
             var sb = StringBuilderPool.Acquire();
-            sb.Append('\'').Append(_values[0]).Append('\'');
+            sb.Append('\'');
+            sb.Append(_values[0]);
+            sb.Append('\'');
             if (_values.Length > 1)
             {
                 for (int i = 1; i < _values.Length - 1; i++)
                 {
-                    sb.Append(", '").Append(_values[i]).Append('\'');
+                    sb.Append(", '");
+                    sb.Append(_values[i]);
+                    sb.Append('\'');
                 }
 
-                sb.Append(" and '").Append(_values[_values.Length - 1]).Append('\'');
+                sb.Append(" and '");
+                sb.Append(_values[_values.Length - 1]);
+                sb.Append('\'');
             }
 
             string valueString = StringBuilderPool.GetValueAndRelease(sb);
diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs
index 6de68d1a7..e7074b6eb 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeAbsentConditionToValue.cs
@@ -56,15 +56,21 @@ public AttributeAbsentConditionToValue(OpenXmlQualifiedName absentAttribute, Ope
                 if (AttributeValueEquals(conditionAttribute.Value, value, false))
                 {
                     var sb = StringBuilderPool.Acquire();
-                    sb.Append('\'').Append(_values[0]).Append('\'');
+                    sb.Append('\'');
+                    sb.Append(_values[0]);
+                    sb.Append('\'');
                     if (_values.Length > 1)
                     {
                         for (int i = 1; i < _values.Length - 1; i++)
                         {
-                            sb.Append(", '").Append(_values[i]).Append('\'');
+                            sb.Append(", '");
+                            sb.Append(_values[i]);
+                            sb.Append('\'');
                         }
 
-                        sb.Append(" or '").Append(_values[_values.Length - 1]).Append('\'');
+                        sb.Append(" or '");
+                        sb.Append(_values[_values.Length - 1]);
+                        sb.Append('\'');
                     }
 
                     string valueString = StringBuilderPool.GetValueAndRelease(sb);
diff --git a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs
index a699dcde9..7c2ff8071 100644
--- a/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs
+++ b/src/DocumentFormat.OpenXml.Framework/Validation/Semantic/AttributeValueConditionToAnother.cs
@@ -66,29 +66,41 @@ public AttributeValueConditionToAnother(OpenXmlQualifiedName attribute, OpenXmlQ
                 if (AttributeValueEquals(conditionAttribute.Value, value, false))
                 {
                     var sb = StringBuilderPool.Acquire();
-                    sb.Append('\'').Append(_values[0]).Append('\'');
+                    sb.Append('\'');
+                    sb.Append(_values[0]);
+                    sb.Append('\'');
                     if (_values.Length > 1)
                     {
                         for (int i = 1; i < _values.Length - 1; i++)
                         {
-                            sb.Append(", '").Append(_values[i]).Append('\'');
+                            sb.Append(", '");
+                            sb.Append(_values[i]);
+                            sb.Append('\'');
                         }
 
-                        sb.Append(" or '").Append(_values[_values.Length - 1]).Append('\'');
+                        sb.Append(" or '");
+                        sb.Append(_values[_values.Length - 1]);
+                        sb.Append('\'');
                     }
 
                     string attributeValueString = StringBuilderPool.GetValueAndRelease(sb);
 
                     var otherSb = StringBuilderPool.Acquire();
-                    otherSb.Append('\'').Append(_otherValues[0]).Append('\'');
+                    otherSb.Append('\'');
+                    otherSb.Append(_otherValues[0]);
+                    otherSb.Append('\'');
                     if (_otherValues.Length > 1)
                     {
                         for (int i = 1; i < _otherValues.Length - 1; i++)
                         {
-                            otherSb.Append(", '").Append(_otherValues[i]).Append('\'');
+                            otherSb.Append(", '");
+                            otherSb.Append(_otherValues[i]);
+                            otherSb.Append('\'');
                         }
 
-                        otherSb.Append(" or '").Append(_otherValues[_otherValues.Length - 1]).Append('\'');
+                        otherSb.Append(" or '");
+                        otherSb.Append(_otherValues[_otherValues.Length - 1]);
+                        otherSb.Append('\'');
                     }
 
                     string otherAttributeValueString = StringBuilderPool.GetValueAndRelease(otherSb);

From 84dad9cef821092bd942fb0de75f6ca171011268 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 28 Apr 2026 03:41:25 +0000
Subject: [PATCH 7/7] Fix CS0122: use new StringBuilder() in HexStringFactory
 instead of internal StringBuilderPool

Agent-Logs-Url: https://github.com/dotnet/Open-XML-SDK/sessions/dba14ab9-b782-474a-8a60-32881273ee7a

Co-authored-by: twsouthwick <583206+twsouthwick@users.noreply.github.com>
---
 .../SimpleTypes/HexStringFactory.cs                        | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
index 75a458006..d321ee52d 100644
--- a/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
+++ b/src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs
@@ -1,8 +1,8 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using DocumentFormat.OpenXml.Framework;
 using System;
+using System.Text;
 
 namespace DocumentFormat.OpenXml
 {
@@ -40,8 +40,7 @@ static string Create(ReadOnlySpan<byte> bytes)
 
             return new string(chars);
 #else
-            var sb = StringBuilderPool.Acquire();
-            sb.EnsureCapacity(bytes.Length * 2);
+            var sb = new StringBuilder(bytes.Length * 2);
 
             foreach (var b in bytes)
             {
@@ -49,7 +48,7 @@ static string Create(ReadOnlySpan<byte> bytes)
                 sb.Append(ToCharUpper(b));
             }
 
-            return StringBuilderPool.GetValueAndRelease(sb);
+            return sb.ToString();
 #endif
 
             static char ToCharUpper(int value)