Eliminate allocation in StreamAsIStream.Read. (#6632)

The native/managed interop layer will allocate a byte array to marshal a native buffer to the StreamDescriptor.Read delegate. By using Span<byte> and the corresponding overload of Stream.Read, we can read directly into the caller's buffer when CManagedStream::Read calls this delegate.

Author: bgrainger

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/StreamAsIStream.cs

@@ -10,6 +10,7 @@
 using System;
 using MS.Internal;
 using MS.Win32;
+using System.Buffers;
 using System.Reflection;
 using System.Collections;
 using System.Diagnostics;
@@ -27,7 +28,7 @@ namespace System.Windows.Media
     internal struct StreamDescriptor
     {
         internal delegate void Dispose(ref StreamDescriptor pSD);
-        internal delegate int Read(ref StreamDescriptor pSD, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 2), Out]byte[] buffer, uint cb, out uint cbRead);
+        internal delegate int Read(ref StreamDescriptor pSD, IntPtr buffer, uint cb, out uint cbRead);
 
         internal unsafe delegate int Seek(ref StreamDescriptor pSD, long offset, uint origin, long* plibNewPostion);
         internal delegate int Stat(ref StreamDescriptor pSD, out System.Runtime.InteropServices.ComTypes.STATSTG statstg, uint grfStatFlag);
@@ -236,7 +237,7 @@ public int CopyTo(IntPtr /* IStream */ pstm, long cb, out long cbRead, out long
 
                     uint read = 0;
 
-                    hr = Read(buffer, toRead, out read);
+                    hr = Read(buffer.AsSpan(0, (int) toRead), out read);
 
                     if (read == 0)
                     {
@@ -289,7 +290,7 @@ public int LockRegion(long libOffset, long cb, uint dwLockType)
             return NativeMethods.E_NOTIMPL;
         }
 
-        public int Read(byte[] buffer, uint cb, out uint cbRead)
+        public int Read(Span<byte> buffer, out uint cbRead)
         {
             cbRead = 0;
 
@@ -300,7 +301,7 @@ public int Read(byte[] buffer, uint cb, out uint cbRead)
                 Verify();
                 ActualizeVirtualPosition();
 
-                cbRead = (uint) dataStream.Read(buffer, 0, (int) cb);
+                cbRead = (uint) dataStream.Read(buffer);
             }
             catch (Exception e)
             {
@@ -596,9 +597,10 @@ internal static int LockRegion(ref StreamDescriptor pSD, long libOffset, long cb
             return (StreamAsIStream.FromSD(ref pSD)).LockRegion(libOffset, cb, dwLockType);
         }
 
-        internal static int Read(ref StreamDescriptor pSD, byte[] buffer, uint cb, out uint cbRead)
+        internal static unsafe int Read(ref StreamDescriptor pSD, IntPtr buffer, uint cb, out uint cbRead)
         {
-            return (StreamAsIStream.FromSD(ref pSD)).Read(buffer, cb, out cbRead);
+            var span = new Span<byte>(buffer.ToPointer(), (int) cb);
+            return (StreamAsIStream.FromSD(ref pSD)).Read(span, out cbRead);
         }
 
         internal static int Revert(ref StreamDescriptor pSD)