Get image data of FrameworkElement in PNG format.

WindowHelper.cs

public static class WindowHelper
{
    public static byte[] GetImageData(FrameworkElement element)
    {
        var dpi = VisualTreeHelper.GetDpi(element);

        var rtb = new RenderTargetBitmap(
            (int)(element.ActualWidth * dpi.DpiScaleX),
            (int)(element.ActualHeight * dpi.DpiScaleY),
            dpi.PixelsPerInchX,
            dpi.PixelsPerInchY,
            PixelFormats.Pbgra32);

        rtb.Render(element);

        var encoder = new PngBitmapEncoder();
        encoder.Frames.Add(BitmapFrame.Create(rtb));

        using var ms = new MemoryStream();
        encoder.Save(ms);
        return ms.ToArray();
    }
}