From 2268e6bd12b5e470f2ca319be0a49a30f534a241 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Mon, 26 Feb 2024 20:29:38 -0500
Subject: [PATCH] Special-case arrays in Enumerable.DefaultIfEmpty

Arrays are fixed-length, and DefaultIfEmpty only applies special behaviors on top of the input enumerable if it's empty. Thus, we can special-case arrays, such that if the input is a non-empty array, we just return the original input rather than wrapping it in a new enumerable.
---
 src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.cs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.cs b/src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.cs
index 593a6b8a67b3d1..b4bef955c0784f 100644
--- a/src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.cs
+++ b/src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.cs
@@ -18,6 +18,11 @@ public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSou
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
+            if (source is TSource[] { Length: > 0 })
+            {
+                return source;
+            }
+
             return new DefaultIfEmptyIterator<TSource>(source, defaultValue);
         }