Avoid repeated delegate allocations in FileStream.FlushAsync (#8048)

Avoid the `Action<object>` allocation on each invocation of
`FileStream.FlushAsync` on Windows. This change makes the Win32
implementation match Unix.

diff --git a/src/mscorlib/corefx/System/IO/FileStream.Win32.cs b/src/mscorlib/corefx/System/IO/FileStream.Win32.cs
index 8b9adb54366b133f2be648fd93047f8e47eef578..350d948b00c8854d1f5acee57febc8470b54b71f 100644 (file)
--- a/src/mscorlib/corefx/System/IO/FileStream.Win32.cs
+++ b/src/mscorlib/corefx/System/IO/FileStream.Win32.cs
@@ -1715,7 +1715,7 @@ namespace System.IO
             if (CanWrite)
             {
                 return Task.Factory.StartNew(
-                    state => FlushOSBuffer(),
+                    state => ((FileStream)state).FlushOSBuffer(),
                     this,
                     cancellationToken,
                     TaskCreationOptions.DenyChildAttach,