From 5a723128c22b42050a451c9c4ac6bfcec996a2f2 Mon Sep 17 00:00:00 2001
From: Cory Nelson <cnelson@ctlglobalsolutions.com>
Date: Tue, 17 Nov 2015 14:55:29 -0600
Subject: [PATCH 1/3] Optimize Enumerable.Skip for IList.

---
 src/System.Linq/src/System/Linq/Enumerable.cs | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/System.Linq/src/System/Linq/Enumerable.cs b/src/System.Linq/src/System/Linq/Enumerable.cs
index 9ed88232d8a2..0c4c5cca325a 100644
--- a/src/System.Linq/src/System/Linq/Enumerable.cs
+++ b/src/System.Linq/src/System/Linq/Enumerable.cs
@@ -884,7 +884,22 @@ private static IEnumerable<TSource> TakeWhileIterator<TSource>(IEnumerable<TSour
         public static IEnumerable<TSource> Skip<TSource>(this IEnumerable<TSource> source, int count)
         {
             if (source == null) throw Error.ArgumentNull("source");
-            return SkipIterator<TSource>(source, count);
+
+            IList<TSource> sourceList = source as IList<TSource>;
+            return sourceList != null ? SkipList(sourceList, count) : SkipIterator<TSource>(source, count);
+        }
+
+        private static IEnumerable<TSource> SkipList<TSource>(IList<TSource> source, int count)
+        {
+            if (count < 0)
+            {
+                count = 0;
+            }
+
+            while (count < source.Count)
+            {
+                yield return source[count++];
+            }
         }
 
         private static IEnumerable<TSource> SkipIterator<TSource>(IEnumerable<TSource> source, int count)

From c9953f400dc80d6ca7d918d2e8cd8c1d43c0ede2 Mon Sep 17 00:00:00 2001
From: Cory Nelson <cnelson@ctlglobalsolutions.com>
Date: Tue, 17 Nov 2015 16:45:52 -0600
Subject: [PATCH 2/3] Avoid repeated interface call as suggested by
 @stephentoub.

---
 src/System.Linq/src/System/Linq/Enumerable.cs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/System.Linq/src/System/Linq/Enumerable.cs b/src/System.Linq/src/System/Linq/Enumerable.cs
index 0c4c5cca325a..9900832e6672 100644
--- a/src/System.Linq/src/System/Linq/Enumerable.cs
+++ b/src/System.Linq/src/System/Linq/Enumerable.cs
@@ -896,7 +896,9 @@ private static IEnumerable<TSource> SkipList<TSource>(IList<TSource> source, int
                 count = 0;
             }
 
-            while (count < source.Count)
+            int sourceCount = source.Count;
+
+            while (count < sourceCount)
             {
                 yield return source[count++];
             }

From 7d775acd6db8e23b22bc976dd52ad336d31e5583 Mon Sep 17 00:00:00 2001
From: Cory Nelson <cnelson@ctlglobalsolutions.com>
Date: Thu, 19 Nov 2015 10:16:22 -0600
Subject: [PATCH 3/3] Revert "Avoid repeated interface call as suggested by
 @stephentoub."

This reverts commit c9953f400dc80d6ca7d918d2e8cd8c1d43c0ede2.
---
 src/System.Linq/src/System/Linq/Enumerable.cs | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/System.Linq/src/System/Linq/Enumerable.cs b/src/System.Linq/src/System/Linq/Enumerable.cs
index 9900832e6672..0c4c5cca325a 100644
--- a/src/System.Linq/src/System/Linq/Enumerable.cs
+++ b/src/System.Linq/src/System/Linq/Enumerable.cs
@@ -896,9 +896,7 @@ private static IEnumerable<TSource> SkipList<TSource>(IList<TSource> source, int
                 count = 0;
             }
 
-            int sourceCount = source.Count;
-
-            while (count < sourceCount)
+            while (count < source.Count)
             {
                 yield return source[count++];
             }