Implemented optimization to only construct a dictionary for priority levels if there is more than one in use. This drops memory consumption in most applications by a reasonable amount since most don't use multiple bindings on single properties (like animation and a local binding).

src/Avalonia.Base/Avalonia.Base.csproj

@@ -118,6 +118,7 @@
     <Compile Include="Utilities\ExceptionUtilities.cs" />
     <Compile Include="Utilities\IWeakSubscriber.cs" />
     <Compile Include="Utilities\MathUtilities.cs" />
+    <Compile Include="Utilities\SingleOrDictionary.cs" />
     <Compile Include="Utilities\TypeUtilities.cs" />
     <Compile Include="Utilities\WeakObservable.cs" />
     <Compile Include="Utilities\WeakSubscriptionManager.cs" />

src/Avalonia.Base/PriorityValue.cs

@@ -28,7 +28,7 @@ namespace Avalonia
     {
         private readonly IPriorityValueOwner _owner;
         private readonly Type _valueType;
-        private readonly Dictionary<int, PriorityLevel> _levels = new Dictionary<int, PriorityLevel>();
+        private readonly SingleOrDictionary<int, PriorityLevel> _levels = new SingleOrDictionary<int, PriorityLevel>();
         private object _value;
         private readonly Func<object, object> _validate;
 

src/Avalonia.Base/Utilities/SingleOrDictionary.cs

@@ -0,0 +1,147 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Avalonia.Utilities
+{
+    /// <summary>
+    /// Stores either a single key value pair or constructs a dictionary when more than one value is stored.
+    /// </summary>
+    /// <typeparam name="TKey">The type of the key.</typeparam>
+    /// <typeparam name="TValue">The type of the value.</typeparam>
+    public class SingleOrDictionary<TKey, TValue> : IEnumerable<KeyValuePair<TKey, TValue>>
+    {
+        private KeyValuePair<TKey, TValue>? singleValue;
+        private Dictionary<TKey, TValue> dictionary;
+
+        private bool useDictionary = false;
+        
+        public void Add(TKey key, TValue value)
+        {
+            if (singleValue != null)
+            {
+                dictionary = new Dictionary<TKey, TValue>();
+                ((ICollection<KeyValuePair<TKey, TValue>>)dictionary).Add(singleValue.Value);
+                useDictionary = true;
+                singleValue = null;
+            }
+
+            if (useDictionary)
+            {
+                dictionary.Add(key, value);
+            }
+            else
+            {
+                singleValue = new KeyValuePair<TKey, TValue>(key, value);
+            }
+        }
+
+        public bool TryGetValue(TKey key, out TValue value)
+        {
+            if (!useDictionary)
+            {
+                if (!singleValue.HasValue || !singleValue.Value.Key.Equals(key))
+                {
+                    value = default(TValue);
+                    return false;
+                }
+                else
+                {
+                    value = singleValue.Value.Value;
+                    return true;
+                }
+            }
+            else
+            {
+                return dictionary.TryGetValue(key, out value);
+            }
+        }
+
+        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
+        {
+            if (!useDictionary)
+            {
+                if (singleValue.HasValue)
+                {
+                    return new SingleEnumerator<KeyValuePair<TKey, TValue>>(singleValue.Value);
+                }
+            }
+            else
+            {
+                return dictionary.GetEnumerator();
+            }
+            return Enumerable.Empty<KeyValuePair<TKey, TValue>>().GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+
+        public IEnumerable<TValue> Values
+        {
+            get
+            {
+                if(!useDictionary)
+                {
+                    if (singleValue.HasValue)
+                    {
+                        return new[] { singleValue.Value.Value };
+                    }
+                }
+                else
+                {
+                    return dictionary.Values;
+                }
+                return Enumerable.Empty<TValue>();
+            }
+        }
+
+        private class SingleEnumerator<T> : IEnumerator<T>
+        {
+            private T value;
+            private int index = -1;
+
+            public SingleEnumerator(T value)
+            {
+                this.value = value;
+            }
+
+            public T Current
+            {
+                get
+                {
+                    if (index == 0)
+                    {
+                        return value;
+                    }
+                    else
+                    {
+                        throw new InvalidOperationException();
+                    }
+                }
+            }
+
+            object IEnumerator.Current => Current;
+
+            public void Dispose()
+            {
+            }
+
+            public bool MoveNext()
+            {
+                index++;
+                return index < 1;
+            }
+
+            public void Reset()
+            {
+                index = -1;
+            }
+        }
+
+    }
+}