Avoid boxing booleans and allocations in UncommonField<bool>

src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/UncommonField.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.
 // See the LICENSE file in the project root for more information.
 
@@ -10,56 +10,66 @@ namespace System.Windows
     internal class UncommonField<T>
     {
         /// <summary>
-        ///     Create a new UncommonField.
+        /// Zero-based, globally unique index, retrieved via <see cref="DependencyProperty.GetUniqueGlobalIndex"/>.
         /// </summary>
-        public UncommonField() : this(default(T))
+        internal int GlobalIndex { get; }
+
+        /// <summary>
+        /// Determines the default value for this field, assigned during initial construction.
+        /// </summary>
+        private readonly T _defaultValue;
+        /// <summary>
+        /// An optimization detail representing whether a non-default value is currently stored.
+        /// </summary>
+        private bool _hasBeenSet;
+
+        /// <summary>
+        /// Create a new UncommonField. Stores <see langword="default"/>(<typeparamref name="T"/>) value as default.
+        /// </summary>
+        public UncommonField() : this(default)
         {
         }
 
         /// <summary>
-        ///     Create a new UncommonField.
+        /// Create a new UncommonField.
         /// </summary>
         /// <param name="defaultValue">The default value of the field.</param>
         public UncommonField(T defaultValue)
         {
             _defaultValue = defaultValue;
-            _hasBeenSet = false;
 
             lock (DependencyProperty.Synchronized)
             {
-                _globalIndex = DependencyProperty.GetUniqueGlobalIndex(null, null);
+                GlobalIndex = DependencyProperty.GetUniqueGlobalIndex(null, null);
 
                 DependencyProperty.RegisteredPropertyList.Add();
             }
         }
 
         /// <summary>
-        ///     Write the given value onto a DependencyObject instance.
+        /// Write the given value onto a DependencyObject instance.
         /// </summary>
         /// <param name="instance">The DependencyObject on which to set the value.</param>
         /// <param name="value">The value to set.</param>
         public void SetValue(DependencyObject instance, T value)
         {
             ArgumentNullException.ThrowIfNull(instance);
 
-            EntryIndex entryIndex = instance.LookupEntry(_globalIndex);
+            EntryIndex entryIndex = instance.LookupEntry(GlobalIndex);
 
-            // Set the value if it's not the default, otherwise remove the value.
-            if (!object.ReferenceEquals(value, _defaultValue))
+            // Special case boolean operations to avoid boxing with (mostly) UIA code paths
+            if (typeof(T) == typeof(bool))
             {
-                object valueObject;
-
-                if (typeof(T) == typeof(bool))
-                {
-                    // Use shared boxed instances rather than creating new objects for each SetValue call.
-                    valueObject = BooleanBoxes.Box(Unsafe.As<T, bool>(ref value));
-                }
-                else
-                {
-                    valueObject = value;
-                }
+                // Use shared boxed instances rather than creating new objects for each SetValue call.
+                object valueObject = BooleanBoxes.Box(Unsafe.BitCast<T, bool>(value));
 
-                instance.SetEffectiveValue(entryIndex, dp: null, _globalIndex, metadata: null, valueObject, BaseValueSourceInternal.Local);
+                instance.SetEffectiveValue(entryIndex, dp: null, GlobalIndex, metadata: null, valueObject, BaseValueSourceInternal.Local);
+                _hasBeenSet = true;
+            }
+            // Set the value if it's not the default, otherwise remove the value.
+            else if (!ReferenceEquals(value, _defaultValue))
+            {
+                instance.SetEffectiveValue(entryIndex, dp: null, GlobalIndex, metadata: null, value, BaseValueSourceInternal.Local);
                 _hasBeenSet = true;
             }
             else
@@ -69,7 +79,7 @@ public void SetValue(DependencyObject instance, T value)
         }
 
         /// <summary>
-        ///     Read the value of this field on a DependencyObject instance.
+        /// Read the value of this field on a DependencyObject instance.
         /// </summary>
         /// <param name="instance">The DependencyObject from which to get the value.</param>
         /// <returns></returns>
@@ -79,7 +89,7 @@ public T GetValue(DependencyObject instance)
 
             if (_hasBeenSet)
             {
-                EntryIndex entryIndex = instance.LookupEntry(_globalIndex);
+                EntryIndex entryIndex = instance.LookupEntry(GlobalIndex);
 
                 if (entryIndex.Found)
                 {
@@ -100,32 +110,16 @@ public T GetValue(DependencyObject instance)
 
 
         /// <summary>
-        ///     Clear this field from the given DependencyObject instance.
+        /// Clear this field from the given DependencyObject instance.
         /// </summary>
-        /// <param name="instance"></param>
+        /// <param name="instance">An instance on which to clear the value from.</param>
         public void ClearValue(DependencyObject instance)
         {
             ArgumentNullException.ThrowIfNull(instance);
 
-            EntryIndex entryIndex = instance.LookupEntry(_globalIndex);
+            EntryIndex entryIndex = instance.LookupEntry(GlobalIndex);
 
             instance.UnsetEffectiveValue(entryIndex, dp: null, metadata: null);
         }
-
-        internal int GlobalIndex
-        {
-            get
-            {
-                return _globalIndex;
-            }
-        }
-
-        #region Private Fields
-
-        private T _defaultValue;
-        private int _globalIndex;
-        private bool _hasBeenSet;
-
-        #endregion
     }
 }