Struct

Author: jkotalik

src/Shared/Dictionary/SmallCapacityDictionary.cs

@@ -12,7 +12,7 @@ namespace Microsoft.AspNetCore.Internal.Dictionary
     /// <summary>
     /// An <see cref="IDictionary{String, Object}"/> type to hold a small amount of items (4 or less in the common case).
     /// </summary>
-    internal class SmallCapacityDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IReadOnlyDictionary<TKey, TValue> where TKey : notnull
+    internal struct SmallCapacityDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IReadOnlyDictionary<TKey, TValue> where TKey : notnull
     {
         // Threshold for size of array to use.
         private static readonly int DefaultArrayThreshold = 4;
@@ -91,17 +91,10 @@ public static SmallCapacityDictionary<TKey, TValue> FromArray(KeyValuePair<TKey,
             {
                 _arrayStorage = items!,
                 _count = start,
+                _comparer = comparer
             };
         }
 
-        /// <summary>
-        /// Creates an empty <see cref="SmallCapacityDictionary{TKey, TValue}"/>.
-        /// </summary>
-        public SmallCapacityDictionary()
-            : this(0, EqualityComparer<TKey>.Default)
-        {
-        }
-
         /// <summary>
         /// Creates a <see cref="SmallCapacityDictionary{TKey, TValue}"/>.
         /// </summary>
@@ -127,6 +120,9 @@ public SmallCapacityDictionary(int capacity)
         /// <param name="comparer">Equality comparison.</param>
         public SmallCapacityDictionary(int capacity, IEqualityComparer<TKey> comparer)
         {
+            _backup = null;
+            _count = 0;
+
             if (comparer is not null && comparer != EqualityComparer<TKey>.Default) // first check for null to avoid forcing default comparer instantiation unnecessarily
             {
                 _comparer = comparer;
@@ -162,6 +158,9 @@ public SmallCapacityDictionary(int capacity, IEqualityComparer<TKey> comparer)
         /// <param name="capacity">Initial capacity.</param>
         public SmallCapacityDictionary(IEnumerable<KeyValuePair<TKey, TValue>> values, int capacity, IEqualityComparer<TKey> comparer)
         {
+            _backup = null;
+            _count = 0;
+
             _comparer = comparer ?? EqualityComparer<TKey>.Default;
 
             _arrayStorage = new KeyValuePair<TKey, TValue>[capacity];
@@ -600,13 +599,18 @@ private static void ThrowArgumentNullExceptionForKey()
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private void EnsureCapacity(int capacity)
+        private void EnsureCapacity(int capacity)   
         {
             EnsureCapacitySlow(capacity);
         }
 
         private void EnsureCapacitySlow(int capacity)
         {
+            if (_arrayStorage == null)
+            {
+                _arrayStorage = Array.Empty<KeyValuePair<TKey, TValue>>();
+            }
+
             if (_arrayStorage.Length < capacity)
             {
                 if (capacity > DefaultArrayThreshold)
@@ -644,7 +648,7 @@ private int FindIndex(TKey key)
 
             for (var i = 0; i < count; i++)
             {
-                if (_comparer.Equals(array[i].Key, key))
+                if (Comparer.Equals(array[i].Key, key))
                 {
                     return i;
                 }
@@ -658,21 +662,26 @@ private bool TryFindItem(TKey key, out TValue? value)
         {
             var array = _arrayStorage;
             var count = _count;
+            value = default;
+
+            if (_arrayStorage == null)
+            {
+                return false;
+            }    
 
             // Elide bounds check for indexing.
             if ((uint)count <= (uint)array.Length)
             {
                 for (var i = 0; i < count; i++)
                 {
-                    if (_comparer.Equals(array[i].Key, key))
+                    if (Comparer.Equals(array[i].Key, key))
                     {
                         value = array[i].Value;
                         return true;
                     }
                 }
             }
 
-            value = default;
             return false;
         }
 
@@ -682,12 +691,17 @@ private bool ContainsKeyArray(TKey key)
             var array = _arrayStorage;
             var count = _count;
 
+            if (array == null)
+            {
+                return false;
+            }    
+
             // Elide bounds check for indexing.
             if ((uint)count <= (uint)array.Length)
             {
                 for (var i = 0; i < count; i++)
                 {
-                    if (_comparer.Equals(array[i].Key, key))
+                    if (Comparer.Equals(array[i].Key, key))
                     {
                         return true;
                     }
@@ -700,7 +714,7 @@ private bool ContainsKeyArray(TKey key)
         /// <inheritdoc />
         public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>
         {
-            private readonly SmallCapacityDictionary<TKey, TValue> _dictionary;
+            private SmallCapacityDictionary<TKey, TValue> _dictionary;
             private int _index;
 
             /// <summary>
@@ -709,11 +723,6 @@ public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>
             /// <param name="dictionary">A <see cref="SmallCapacityDictionary{TKey, TValue}"/>.</param>
             public Enumerator(SmallCapacityDictionary<TKey, TValue> dictionary)
             {
-                if (dictionary == null)
-                {
-                    throw new ArgumentNullException();
-                }
-
                 _dictionary = dictionary;
 
                 Current = default;
@@ -738,7 +747,7 @@ public void Dispose()
             public bool MoveNext()
             {
                 var dictionary = _dictionary;
-                if (dictionary._arrayStorage.Length >= _index)
+                if (dictionary._arrayStorage == null || dictionary._arrayStorage.Length >= _index)
                 {
                     return false;
                 }

src/Shared/test/Shared.Tests/SmallCapacityDictionaryTests.cs

@@ -21,7 +21,7 @@ public void DefaultCtor()
 
             // Assert
             Assert.Empty(dict);
-            Assert.Empty(dict._arrayStorage);
+            Assert.Null(dict._arrayStorage);
             Assert.Null(dict._backup);
         }
 
@@ -34,7 +34,7 @@ public void CreateFromNull()
 
             // Assert
             Assert.Empty(dict);
-            Assert.Empty(dict._arrayStorage);
+            Assert.Null(dict._arrayStorage);
             Assert.Null(dict._backup);
         }
 
@@ -371,7 +371,6 @@ public void Keys_EmptyStorage()
 
             // Assert
             Assert.Empty(keys);
-            Assert.IsType<KeyValuePair<string, object?>[]>(dict._arrayStorage);
         }
 
         [Fact]
@@ -402,7 +401,6 @@ public void Values_EmptyStorage()
 
             // Assert
             Assert.Empty(values);
-            Assert.IsType<KeyValuePair<string, object?>[]>(dict._arrayStorage);
         }
 
         [Fact]
@@ -511,26 +509,6 @@ public void Add_DuplicateKey_CaseInsensitive()
             Assert.IsType<KeyValuePair<string, object?>[]>(dict._arrayStorage);
         }
 
-        [Fact]
-        public void Add_KeyValuePair()
-        {
-            // Arrange
-            var dict = new SmallCapacityDictionary<string, object>()
-            {
-                { "age", 30 },
-            };
-
-            // Act
-            ((ICollection<KeyValuePair<string, object?>>)dict).Add(new KeyValuePair<string, object?>("key", "value"));
-
-            // Assert
-            Assert.Collection(
-                dict.OrderBy(kvp => kvp.Key),
-                kvp => { Assert.Equal("age", kvp.Key); Assert.Equal(30, kvp.Value); },
-                kvp => { Assert.Equal("key", kvp.Key); Assert.Equal("value", kvp.Value); });
-            Assert.IsType<KeyValuePair<string, object?>[]>(dict._arrayStorage);
-        }
-
         [Fact]
         public void Clear_EmptyStorage()
         {
@@ -1246,7 +1224,7 @@ public void ListStorage_SwitchesToDictionaryAfter4()
             storage = Assert.IsType<KeyValuePair<string, object?>[]>(dict._arrayStorage);
             Assert.Equal(4, storage.Length);
 
-            Assert.Equal(8, dict.Count);
+            Assert.Equal(5, dict.Count);
         }
 
         [Fact]