Use pooled StringBuilders to reduce allocations

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
+        }
+    }
+}

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);
             }
         }
 

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

src/DocumentFormat.OpenXml.Framework/SimpleTypes/HexStringFactory.cs

@@ -2,10 +2,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System;
-
-#if !NET6_0_OR_GREATER
 using System.Text;
-#endif
 
 namespace DocumentFormat.OpenXml
 {
@@ -73,6 +70,8 @@ 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>
-        public static string Create(params byte[] bytes) => Create(bytes.AsSpan());
+        // 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));
     }
 }

src/DocumentFormat.OpenXml.Framework/SimpleTypes/ListValue.cs

@@ -1,13 +1,13 @@
 // 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;
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Diagnostics;
-using System.Text;
 
 namespace DocumentFormat.OpenXml
 {
@@ -194,7 +194,7 @@ 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 +207,7 @@ public override string? InnerText
                         }
                     }
 
-                    TextValue = textString.ToString();
+                    TextValue = StringBuilderPool.GetValueAndRelease(textString);
                 }
 
                 return TextValue;

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
 {
@@ -60,19 +59,25 @@ public AttributeAbsentConditionToNonValue(OpenXmlQualifiedName absentAttribute,
                 }
             }
 
-            var sb = new StringBuilder();
-            sb.Append('\'').Append(_values[0]).Append('\'');
+            var sb = StringBuilderPool.Acquire();
+            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 = sb.ToString();
+            string valueString = StringBuilderPool.GetValueAndRelease(sb);
 
             return new ValidationErrorInfo()
             {

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
 {
@@ -56,19 +55,25 @@ public AttributeAbsentConditionToValue(OpenXmlQualifiedName absentAttribute, Ope
             {
                 if (AttributeValueEquals(conditionAttribute.Value, value, false))
                 {
-                    var sb = new StringBuilder();
-                    sb.Append('\'').Append(_values[0]).Append('\'');
+                    var sb = StringBuilderPool.Acquire();
+                    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 = sb.ToString();
+                    string valueString = StringBuilderPool.GetValueAndRelease(sb);
 
                     return new ValidationErrorInfo()
                     {

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)),
             };
         }
     }

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
 {
@@ -66,33 +65,45 @@ public AttributeValueConditionToAnother(OpenXmlQualifiedName attribute, OpenXmlQ
             {
                 if (AttributeValueEquals(conditionAttribute.Value, value, false))
                 {
-                    var sb = new StringBuilder();
-                    sb.Append('\'').Append(_values[0]).Append('\'');
+                    var sb = StringBuilderPool.Acquire();
+                    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 = sb.ToString();
+                    string attributeValueString = StringBuilderPool.GetValueAndRelease(sb);
 
-                    var otherSb = new StringBuilder();
-                    otherSb.Append('\'').Append(_otherValues[0]).Append('\'');
+                    var otherSb = StringBuilderPool.Acquire();
+                    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 = otherSb.ToString();
+                    string otherAttributeValueString = StringBuilderPool.GetValueAndRelease(otherSb);
 
                     return new ValidationErrorInfo()
                     {

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)