Avoid boxing in System.ObjectModel.KeydCollection during startup

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" />

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)
             {

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);
+    }
+}