Where we're going we don't need FileStreams

gistfile1.cs

public Task Write(ArraySegment<byte> buffer)
{
    unsafe
    {
        fixed (byte* thing = buffer.Array) // this fixed expires but the real fixed is that done by Pack (lives until callback)
        {
            var tcs = new TaskCompletionSource<uint>();

            var overlapped = new Overlapped((int)_pos, (int)(_pos>>32), IntPtr.Zero, null);
            _pos += (ulong)buffer.Count;

            NativeMethods.WriteFileChecked(_handle, &thing[buffer.Offset], buffer.Count, IntPtr.Zero, overlapped.Pack(
                (errCode, numBytes, nativeOverlapped) =>
                {
                    Overlapped.Free(nativeOverlapped);
                    if (errCode != 0)
                    {
                        tcs.SetException(new Win32Exception((int)errCode));
                    }
                    else
                    {
                        tcs.SetResult(numBytes);
                    }
                }, buffer.Array));

            return tcs.Task;
        }
    }
}