var screen = System.Windows.Application.Current.Windows.OfType<Window>().SingleOrDefault(x => x.IsActive);

            //Set the desired resolution to 4000 x 4000
            var rtb = new RenderTargetBitmap(4000, 4000, 96, 96, System.Windows.Media.PixelFormats.Pbgra32);

            rtb.Render(screen);

            var enc = new System.Windows.Media.Imaging.PngBitmapEncoder();
            enc.Frames.Add(System.Windows.Media.Imaging.BitmapFrame.Create(rtb));

            using (var stm = System.IO.File.Create("ScreenShot.png"))
            {
                enc.Save(stm);
            }