From 05bbf7dacd2ecb116db664bc638a779a69da0240 Mon Sep 17 00:00:00 2001
From: Ben Adams <thundercat@illyriad.co.uk>
Date: Wed, 25 Sep 2019 01:24:01 +0100
Subject: [PATCH] Add CollectionsMarshal (dotnet/coreclr#26867)

* Add CollectionsMarshal

* Feedback

* Feedback

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>
---
 .../System.Private.CoreLib.Shared.projitems   |  1 +
 .../shared/System/Collections/Generic/List.cs |  4 ++--
 .../InteropServices/CollectionsMarshal.cs     | 21 +++++++++++++++++++
 3 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 src/System.Private.CoreLib/shared/System/Runtime/InteropServices/CollectionsMarshal.cs

diff --git a/src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems b/src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems
index 90d68d4156f..5e8a46b1375 100644
--- a/src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems
+++ b/src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems
@@ -644,6 +644,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ClassInterfaceAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ClassInterfaceType.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CoClassAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CollectionsMarshal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ComDefaultInterfaceAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ComEventInterfaceAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ComImportAttribute.cs" />
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Generic/List.cs b/src/System.Private.CoreLib/shared/System/Collections/Generic/List.cs
index 62993e71e55..ce372c25567 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Generic/List.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Generic/List.cs
@@ -23,8 +23,8 @@ public class List<T> : IList<T>, IList, IReadOnlyList<T>
     {
         private const int DefaultCapacity = 4;
 
-        private T[] _items; // Do not rename (binary serialization)
-        private int _size; // Do not rename (binary serialization)
+        internal T[] _items; // Do not rename (binary serialization)
+        internal int _size; // Do not rename (binary serialization)
         private int _version; // Do not rename (binary serialization)
 
 #pragma warning disable CA1825 // avoid the extra generic instantiation for Array.Empty<T>()
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/CollectionsMarshal.cs b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/CollectionsMarshal.cs
new file mode 100644
index 00000000000..fe9df260ac6
--- /dev/null
+++ b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/CollectionsMarshal.cs
@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+
+namespace System.Runtime.InteropServices
+{
+    /// <summary>
+    /// An unsafe class that provides a set of methods to access the underlying data representations of collections.
+    /// </summary>
+    public static class CollectionsMarshal
+    {
+        /// <summary>
+        /// Get a <see cref="Span{T}"/> view over a <see cref="List{T}"/>'s data.
+        /// Items should not be added or removed from the <see cref="List{T}"/> while the <see cref="Span{T}"/> is in use.
+        /// </summary>
+        public static Span<T> AsSpan<T>(List<T> list)
+            => new Span<T>(list._items, 0, list._size);
+    }
+}