From dc08a60f5edebdf3d6048afb1b147de776661562 Mon Sep 17 00:00:00 2001
From: Ben Adams <thundercat@illyriad.co.uk>
Date: Sun, 14 Apr 2019 16:09:17 +0100
Subject: [PATCH] Lazy allocate dictionary in ItemsDictionary (#9284)

---
 ...Microsoft.AspNetCore.Http.netcoreapp3.0.cs |   2 +-
 src/Http/Http/src/Internal/ItemsDictionary.cs | 128 ++++++++++++------
 2 files changed, 87 insertions(+), 43 deletions(-)

diff --git a/src/Http/Http/ref/Microsoft.AspNetCore.Http.netcoreapp3.0.cs b/src/Http/Http/ref/Microsoft.AspNetCore.Http.netcoreapp3.0.cs
index c2ac47f401..4070311a3b 100644
--- a/src/Http/Http/ref/Microsoft.AspNetCore.Http.netcoreapp3.0.cs
+++ b/src/Http/Http/ref/Microsoft.AspNetCore.Http.netcoreapp3.0.cs
@@ -406,7 +406,7 @@ namespace Microsoft.AspNetCore.Http.Internal
     {
         public ItemsDictionary() { }
         public ItemsDictionary(System.Collections.Generic.IDictionary<object, object> items) { }
-        public System.Collections.Generic.IDictionary<object, object> Items { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public System.Collections.Generic.IDictionary<object, object> Items { get { throw null; } }
         int System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<System.Object,System.Object>>.Count { get { throw null; } }
         bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<System.Object,System.Object>>.IsReadOnly { get { throw null; } }
         object System.Collections.Generic.IDictionary<System.Object,System.Object>.this[object key] { get { throw null; } set { } }
diff --git a/src/Http/Http/src/Internal/ItemsDictionary.cs b/src/Http/Http/src/Internal/ItemsDictionary.cs
index 4821912240..7f5f458ba1 100644
--- a/src/Http/Http/src/Internal/ItemsDictionary.cs
+++ b/src/Http/Http/src/Internal/ItemsDictionary.cs
@@ -8,111 +8,155 @@ namespace Microsoft.AspNetCore.Http.Internal
 {
     public class ItemsDictionary : IDictionary<object, object>
     {
+        private IDictionary<object, object> _items;
+
         public ItemsDictionary()
-            : this(new Dictionary<object, object>())
-        {
-        }
+        {}
 
         public ItemsDictionary(IDictionary<object, object> items)
         {
-            Items = items;
+            _items = items;
         }
 
-        public IDictionary<object, object> Items { get; }
+        public IDictionary<object, object> Items => this;
 
         // Replace the indexer with one that returns null for missing values
         object IDictionary<object, object>.this[object key]
         {
             get
             {
-                object value;
-                if (Items.TryGetValue(key, out value))
+                if (_items != null && _items.TryGetValue(key, out var value))
                 {
                     return value;
                 }
                 return null;
             }
-            set { Items[key] = value; }
+            set
+            {
+                EnsureDictionary();
+                _items[key] = value;
+            }
         }
 
         void IDictionary<object, object>.Add(object key, object value)
         {
-            Items.Add(key, value);
+            EnsureDictionary();
+            _items.Add(key, value);
         }
 
         bool IDictionary<object, object>.ContainsKey(object key)
-        {
-            return Items.ContainsKey(key);
-        }
+            => _items != null && _items.ContainsKey(key);
 
         ICollection<object> IDictionary<object, object>.Keys
         {
-            get { return Items.Keys; }
+            get
+            {
+                if (_items == null)
+                {
+                    return EmptyDictionary.Dictionary.Keys;
+                }
+
+                return _items.Keys;
+            }
         }
 
         bool IDictionary<object, object>.Remove(object key)
-        {
-            return Items.Remove(key);
-        }
+            => _items != null && _items.Remove(key);
 
         bool IDictionary<object, object>.TryGetValue(object key, out object value)
         {
-            return Items.TryGetValue(key, out value);
+            value = null;
+            return _items != null && _items.TryGetValue(key, out value);
         }
 
         ICollection<object> IDictionary<object, object>.Values
         {
-            get { return Items.Values; }
+            get
+            {
+                if (_items == null)
+                {
+                    return EmptyDictionary.Dictionary.Values;
+                }
+
+                return _items.Values;
+            }
         }
 
         void ICollection<KeyValuePair<object, object>>.Add(KeyValuePair<object, object> item)
         {
-            Items.Add(item);
+            EnsureDictionary();
+            _items.Add(item);
         }
 
-        void ICollection<KeyValuePair<object, object>>.Clear()
-        {
-            Items.Clear();
-        }
+        void ICollection<KeyValuePair<object, object>>.Clear() => _items?.Clear();
 
         bool ICollection<KeyValuePair<object, object>>.Contains(KeyValuePair<object, object> item)
-        {
-            return Items.Contains(item);
-        }
+            => _items != null && _items.Contains(item);
 
         void ICollection<KeyValuePair<object, object>>.CopyTo(KeyValuePair<object, object>[] array, int arrayIndex)
         {
-            Items.CopyTo(array, arrayIndex);
+            if (_items == null)
+            {
+                //Delegate to Empty Dictionary to do the argument checking.
+                EmptyDictionary.Collection.CopyTo(array, arrayIndex);
+            }
+
+            _items.CopyTo(array, arrayIndex);
         }
 
-        int ICollection<KeyValuePair<object, object>>.Count
-        {
-            get { return Items.Count; }
-        }
+        int ICollection<KeyValuePair<object, object>>.Count => _items?.Count ?? 0;
 
-        bool ICollection<KeyValuePair<object, object>>.IsReadOnly
-        {
-            get { return Items.IsReadOnly; }
-        }
+        bool ICollection<KeyValuePair<object, object>>.IsReadOnly => _items?.IsReadOnly ?? false;
 
         bool ICollection<KeyValuePair<object, object>>.Remove(KeyValuePair<object, object> item)
         {
-            object value;
-            if (Items.TryGetValue(item.Key, out value) && Equals(item.Value, value))
+            if (_items == null)
             {
-                return Items.Remove(item.Key);
+                return false;
+            }
+
+            if (_items.TryGetValue(item.Key, out var value) && Equals(item.Value, value))
+            {
+                return _items.Remove(item.Key);
             }
             return false;
         }
 
-        IEnumerator<KeyValuePair<object, object>> IEnumerable<KeyValuePair<object, object>>.GetEnumerator()
+        private void EnsureDictionary()
         {
-            return Items.GetEnumerator();
+            if (_items == null)
+            {
+                _items = new Dictionary<object, object>();
+            }
         }
 
-        IEnumerator IEnumerable.GetEnumerator()
+        IEnumerator<KeyValuePair<object, object>> IEnumerable<KeyValuePair<object, object>>.GetEnumerator()
+            => _items?.GetEnumerator() ?? EmptyEnumerator.Instance;
+
+        IEnumerator IEnumerable.GetEnumerator() => _items.GetEnumerator() ?? EmptyEnumerator.Instance;
+
+        private class EmptyEnumerator : IEnumerator<KeyValuePair<object, object>>
         {
-            return Items.GetEnumerator();
+            // In own class so only initalized if GetEnumerator is called on an empty ItemsDictionary
+            public readonly static IEnumerator<KeyValuePair<object, object>> Instance = new EmptyEnumerator();
+            public KeyValuePair<object, object> Current => default;
+
+            object IEnumerator.Current => null;
+
+            public void Dispose()
+            { }
+
+            public bool MoveNext() => false;
+
+            public void Reset()
+            { }
+        }
+
+        private static class EmptyDictionary
+        {
+            // In own class so only initalized if CopyTo is called on an empty ItemsDictionary
+            public readonly static IDictionary<object, object> Dictionary = new Dictionary<object, object>();
+            public static ICollection<KeyValuePair<object, object>> Collection => Dictionary;
         }
     }
 }
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