Implement simple struct wrapping a single value or List.

src/Avalonia.Base/Utilities/ValueSingleOrList.cs

@@ -0,0 +1,94 @@
+using System.Collections.Generic;
+
+namespace Avalonia.Utilities
+{
+    /// <summary>
+    /// A list like struct optimized for holding zero or one items.
+    /// </summary>
+    /// <typeparam name="T">The type of items held in the list.</typeparam>
+    /// <remarks>
+    /// Once more than value has been added to this storage it will switch to using <see cref="List"/> internally.
+    /// </remarks>
+    public ref struct ValueSingleOrList<T>
+    {
+        private bool _isSingleSet;
+
+        /// <summary>
+        /// Single contained value. Only valid if <see cref="IsSingle"/> is set.
+        /// </summary>
+        public T Single { get; private set; }
+
+        /// <summary>
+        /// List of values.
+        /// </summary>
+        public List<T> List { get; private set; }
+
+        /// <summary>
+        /// If this struct is backed by a list.
+        /// </summary>
+        public bool HasList => List != null;
+
+        /// <summary>
+        /// If this struct contains only single value and storage was not promoted to a list.
+        /// </summary>
+        public bool IsSingle => List == null && _isSingleSet;
+
+        /// <summary>
+        /// Adds a value.
+        /// </summary>
+        /// <param name="value">Value to add.</param>
+        public void Add(T value)
+        {
+            if (List != null)
+            {
+                List.Add(value);
+            }
+            else
+            {
+                if (!_isSingleSet)
+                {
+                    Single = value;
+
+                    _isSingleSet = true;
+                }
+                else
+                {
+                    List = new List<T>();
+
+                    List.Add(Single);
+                    List.Add(value);
+
+                    Single = default;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Removes a value.
+        /// </summary>
+        /// <param name="value">Value to remove.</param>
+        public bool Remove(T value)
+        {
+            if (List != null)
+            {
+                return List.Remove(value);
+            }
+
+            if (!_isSingleSet)
+            {
+                return false;
+            }
+
+            if (EqualityComparer<T>.Default.Equals(Single, value))
+            {
+                Single = default;
+
+                _isSingleSet = false;
+
+                return true;
+            }
+
+            return false;
+        }
+    }
+}

src/Avalonia.Styling/Styling/Selector.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using Avalonia.Utilities;
 
 namespace Avalonia.Styling
 {
@@ -42,8 +43,7 @@ namespace Avalonia.Styling
         /// <returns>A <see cref="SelectorMatch"/>.</returns>
         public SelectorMatch Match(IStyleable control, bool subscribe = true)
         {
-            List<IObservable<bool>> inputs = null;
-            IObservable<bool> singleInput = null;
+            ValueSingleOrList<IObservable<bool>> inputs = default;
 
             var selector = this;
             var alwaysThisType = true;
@@ -71,40 +71,23 @@ namespace Avalonia.Styling
                 {
                     Debug.Assert(match.Activator != null);
 
-                    if (inputs != null)
-                    {
-                        inputs.Add(match.Activator);
-                    }
-                    else
-                    {
-                        if (singleInput == null)
-                        {
-                            singleInput = match.Activator;
-                        }
-                        else
-                        {
-                            inputs = new List<IObservable<bool>>();
-
-                            inputs.Add(singleInput);
-                            inputs.Add(match.Activator);
-                        }
-                    }
+                    inputs.Add(match.Activator);
                 }
 
                 selector = selector.MovePrevious();
             }
 
-            if (inputs != null)
+            if (inputs.HasList)
             {
-                return new SelectorMatch(StyleActivator.And(inputs));
+                return new SelectorMatch(StyleActivator.And(inputs.List));
             }
-            else if (singleInput != null)
+            else if (inputs.IsSingle)
             {
-                return new SelectorMatch(singleInput);
+                return new SelectorMatch(inputs.Single);
             }
             else
             {
-                return alwaysThisType && !hitCombinator ? 
+                return alwaysThisType && !hitCombinator ?
                     SelectorMatch.AlwaysThisType :
                     SelectorMatch.AlwaysThisInstance;
             }