Split MemoryMarshal.Portable.cs

Author: jkotas

src/System.Memory/src/System.Memory.csproj

@@ -114,6 +114,7 @@
     <Compile Include="System\SpanHelpers.cs" />
     <Compile Include="System\Pinnable.cs" />
     <Compile Include="System\SpanHelpers.Clear.cs" />
+    <Compile Include="System\Runtime\InteropServices\MemoryMarshal.Portable.cs" />
   </ItemGroup>
   <ItemGroup>
     <!-- Common or Common-branched source files -->

src/System.Memory/src/System/Runtime/InteropServices/MemoryMarshal.Portable.cs

@@ -0,0 +1,51 @@
+// 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.Runtime.CompilerServices;
+
+namespace System.Runtime.InteropServices
+{
+    /// <summary>
+    /// Provides a collection of methods for interoperating with <see cref="Memory{T}"/>, <see cref="ReadOnlyMemory{T}"/>,
+    /// <see cref="Span{T}"/>, and <see cref="ReadOnlySpan{T}"/>.
+    /// </summary>
+    public static partial class MemoryMarshal
+    {
+        /// <summary>Creates a <see cref="Memory{T}"/> from a <see cref="ReadOnlyMemory{T}"/>.</summary>
+        /// <param name="readOnlyMemory">The <see cref="ReadOnlyMemory{T}"/>.</param>
+        /// <returns>A <see cref="Memory{T}"/> representing the same memory as the <see cref="ReadOnlyMemory{T}"/>, but writable.</returns>
+        /// <remarks>
+        /// <see cref="AsMemory{T}(ReadOnlyMemory{T})"/> must be used with extreme caution.  <see cref="ReadOnlyMemory{T}"/> is used
+        /// to represent immutable data and other memory that is not meant to be written to; <see cref="Memory{T}"/> instances created
+        /// by <see cref="AsMemory{T}(ReadOnlyMemory{T})"/> should not be written to.  The method exists to enable variables typed
+        /// as <see cref="Memory{T}"/> but only used for reading to store a <see cref="ReadOnlyMemory{T}"/>.
+        /// </remarks>
+        public static Memory<T> AsMemory<T>(ReadOnlyMemory<T> readOnlyMemory) =>
+            Unsafe.As<ReadOnlyMemory<T>, Memory<T>>(ref readOnlyMemory);
+
+        /// <summary>
+        /// Returns a reference to the 0th element of the Span. If the Span is empty, returns a reference to the location where the 0th element
+        /// would have been stored. Such a reference can be used for pinning but must never be dereferenced.
+        /// </summary>
+        public static ref T GetReference<T>(Span<T> span)
+        {
+            if (span.Pinnable == null)
+                unsafe { return ref Unsafe.AsRef<T>(span.ByteOffset.ToPointer()); }
+            else
+                return ref Unsafe.AddByteOffset<T>(ref span.Pinnable.Data, span.ByteOffset);
+        }
+
+        /// <summary>
+        /// Returns a reference to the 0th element of the ReadOnlySpan. If the Span is empty, returns a reference to the location where the 0th element
+        /// would have been stored. Such a reference can be used for pinning but must never be dereferenced.
+        /// </summary>
+        public static ref T GetReference<T>(ReadOnlySpan<T> span)
+        {
+            if (span.Pinnable == null)
+                unsafe { return ref Unsafe.AsRef<T>(span.ByteOffset.ToPointer()); }
+            else
+                return ref Unsafe.AddByteOffset<T>(ref span.Pinnable.Data, span.ByteOffset);
+        }
+    }
+}