SaveWindowContent(this, "c:\\tmp.bmp");

        private void SaveWindowContent(Window source, string fileName)
        {
            FrameworkElement elem = source.Content as FrameworkElement;
            RenderTargetBitmap targetBitmap = new RenderTargetBitmap(
                                       (int)elem.ActualWidth,
                                       (int)elem.ActualHeight,
                                       96d,
                                       96d,
                                       PixelFormats.Default);

            targetBitmap.Render(source);
            BmpBitmapEncoder encoder = new BmpBitmapEncoder();
            encoder.Frames.Add(BitmapFrame.Create(targetBitmap));
            // save file to disk
            using (FileStream fs = File.Open(fileName, FileMode.OpenOrCreate))
            {
                encoder.Save(fs);
            }
        }