From 44cb7bff485f32c8cabdaf0a00298b56be4914b1 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Tue, 2 Jul 2019 22:53:36 -0400
Subject: [PATCH] Avoid extra allocation and interface calls in Append
 (dotnet/corefx#39045)

Commit migrated from https://github.com/dotnet/corefx/commit/2ead5143ce31d6d25089a1f16e9619a374d48d4b
---
 .../System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs     |  8 +++-----
 src/libraries/System.Linq/src/System/Linq/AppendPrepend.cs    |  2 +-
 src/libraries/System.Linq/src/System/Linq/SingleLinkedNode.cs | 11 +----------
 3 files changed, 5 insertions(+), 16 deletions(-)

diff --git a/src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs
index a8d3688..a7d3532 100644
--- a/src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs
+++ b/src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs
@@ -181,19 +181,17 @@ namespace System.Linq
             {
                 int count = GetCount(onlyIfCheap: true);
                 List<TSource> list = count == -1 ? new List<TSource>() : new List<TSource>(count);
+
                 for (SingleLinkedNode<TSource> node = _prepended; node != null; node = node.Linked)
                 {
                     list.Add(node.Item);
                 }
 
                 list.AddRange(_source);
+
                 if (_appended != null)
                 {
-                    IEnumerator<TSource> e = _appended.GetEnumerator(_appendCount);
-                    while (e.MoveNext())
-                    {
-                        list.Add(e.Current);
-                    }
+                    list.AddRange(_appended.ToArray(_appendCount));
                 }
 
                 return list;
diff --git a/src/libraries/System.Linq/src/System/Linq/AppendPrepend.cs b/src/libraries/System.Linq/src/System/Linq/AppendPrepend.cs
index a9bf0a6..08fe6af 100644
--- a/src/libraries/System.Linq/src/System/Linq/AppendPrepend.cs
+++ b/src/libraries/System.Linq/src/System/Linq/AppendPrepend.cs
@@ -220,7 +220,7 @@ namespace System.Linq
                             return false;
                         }
 
-                        _enumerator = _appended.GetEnumerator(_appendCount);
+                        _enumerator = ((IEnumerable<TSource>)_appended.ToArray(_appendCount)).GetEnumerator();
                         _state = 4;
                         goto case 4;
                     case 4:
diff --git a/src/libraries/System.Linq/src/System/Linq/SingleLinkedNode.cs b/src/libraries/System.Linq/src/System/Linq/SingleLinkedNode.cs
index dd85bd8..fdcb433 100644
--- a/src/libraries/System.Linq/src/System/Linq/SingleLinkedNode.cs
+++ b/src/libraries/System.Linq/src/System/Linq/SingleLinkedNode.cs
@@ -65,15 +65,6 @@ namespace System.Linq
         }
 
         /// <summary>
-        /// Gets an <see cref="IEnumerator{TSource}"/> that enumerates the items of this node's singly-linked list in reverse.
-        /// </summary>
-        /// <param name="count">The number of items in this node.</param>
-        public IEnumerator<TSource> GetEnumerator(int count)
-        {
-            return ((IEnumerable<TSource>)ToArray(count)).GetEnumerator();
-        }
-
-        /// <summary>
         /// Gets the node at a logical index by walking the linked list.
         /// </summary>
         /// <param name="index">The logical index.</param>
@@ -98,7 +89,7 @@ namespace System.Linq
         /// Returns an <see cref="T:TSource[]"/> that contains the items of this node's singly-linked list in reverse.
         /// </summary>
         /// <param name="count">The number of items in this node.</param>
-        private TSource[] ToArray(int count)
+        public TSource[] ToArray(int count)
         {
             Debug.Assert(count == GetCount());
 
-- 
2.7.4