From ccc9e231ced3f7ff2e89f2035f5666352f5d3363 Mon Sep 17 00:00:00 2001
From: Jan Kotas <jkotas@microsoft.com>
Date: Sat, 6 Jul 2024 00:07:16 -0700
Subject: [PATCH] Avoid boxing in System.ObjectModel.KeydCollection during
 startup

ArgumentNullException.ThrowIfNull can incur boxing when applied to argument
of generic type. Tier-1 JIT optimizations are able to optimize this boxing
in steady state, but Tier-0 JIT optimization are not. It can result into excessive
allocations during startup. Switch KeyedCollection to use ThrowHelper
that is pattern used by number of other collections.
---
 .../src/System.ObjectModel.csproj                 |  1 +
 .../Collections/ObjectModel/KeyedCollection.cs    | 15 ++++++++++++---
 .../System.ObjectModel/src/System/ThrowHelper.cs  | 14 ++++++++++++++
 3 files changed, 27 insertions(+), 3 deletions(-)
 create mode 100644 src/libraries/System.ObjectModel/src/System/ThrowHelper.cs

diff --git a/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj b/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj
index 0ef93fda9ecd4b..3d787f35f0bbcf 100644
--- a/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj
+++ b/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj
@@ -8,6 +8,7 @@
 
   <ItemGroup>
     <Compile Include="System.ObjectModel.Forwards.cs" />
+    <Compile Include="System\ThrowHelper.cs" />
     <Compile Include="System\Collections\CollectionHelpers.cs" />
     <Compile Include="System\Collections\Generic\DebugView.cs" />
     <Compile Include="System\Collections\Specialized\INotifyCollectionChanged.cs" />
diff --git a/src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/KeyedCollection.cs b/src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/KeyedCollection.cs
index d4fef7be1cb83f..f8e47d71e5e20a 100644
--- a/src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/KeyedCollection.cs
+++ b/src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/KeyedCollection.cs
@@ -71,7 +71,10 @@ public TItem this[TKey key]
 
         public bool Contains(TKey key)
         {
-            ArgumentNullException.ThrowIfNull(key);
+            if (key == null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(key));
+            }
 
             if (dict != null)
             {
@@ -91,7 +94,10 @@ public bool Contains(TKey key)
 
         public bool TryGetValue(TKey key, [MaybeNullWhen(false)] out TItem item)
         {
-            ArgumentNullException.ThrowIfNull(key);
+            if (key == null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(key));
+            }
 
             if (dict != null)
             {
@@ -131,7 +137,10 @@ private bool ContainsItem(TItem item)
 
         public bool Remove(TKey key)
         {
-            ArgumentNullException.ThrowIfNull(key);
+            if (key == null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(key));
+            }
 
             if (dict != null)
             {
diff --git a/src/libraries/System.ObjectModel/src/System/ThrowHelper.cs b/src/libraries/System.ObjectModel/src/System/ThrowHelper.cs
new file mode 100644
index 00000000000000..fe453d70771d34
--- /dev/null
+++ b/src/libraries/System.ObjectModel/src/System/ThrowHelper.cs
@@ -0,0 +1,14 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics.CodeAnalysis;
+
+namespace System
+{
+    internal static class ThrowHelper
+    {
+        [DoesNotReturn]
+        public static void ThrowArgumentNullException(string? paramName) =>
+            throw new ArgumentNullException(paramName);
+    }
+}