ReadAsync.cs

private async Task<byte[]> ReadAsync(string filename)
{
    var fileInfo = new FileInfo(filename);
    using (var stream = new FileStream(filename, FileMode.Open))
    {
        var buffer = new byte[fileInfo.Length];
        await Task<int>.Factory.FromAsync(stream.BeginRead, stream.EndRead, buffer, 0, buffer.Length, null);
        return buffer;
    }
}