WPF 中保存 window(窗口)或者canvas成图片

private void button_save_window_Click(object sender, RoutedEventArgs e)
{

util.SaveWindow(this,96,"d:\\window.png");
}

private void button_save_canvas_Click(object sender, RoutedEventArgs e)
{

util.SaveCanvas(this,this.canvas1, 96, "d:\\canvas.png");
}
}

public static class util
{
public static void SaveWindow(Window window, int dpi, string filename)
{

var rtb = new RenderTargetBitmap(
(int)window.Width, //width
(int)window.Width, //height
dpi, //dpi x
dpi, //dpi y
PixelFormats.Pbgra32 // pixelformat
);
rtb.Render(window);

SaveRTBAsPNG(rtb, filename);

}

public static void SaveCanvas(Window window, Canvas canvas, int dpi, string filename)
{
Size size = new Size(window.Width , window.Height );
canvas.Measure(size);
//canvas.Arrange(new Rect(size));

var rtb = new RenderTargetBitmap(
(int)window.Width, //width
(int)window.Height, //height
dpi, //dpi x
dpi, //dpi y
PixelFormats.Pbgra32 // pixelformat
);
rtb.Render(canvas);

SaveRTBAsPNG(rtb, filename);
}

private static void SaveRTBAsPNG(RenderTargetBitmap bmp, string filename)
{
var enc = new System.Windows.Media.Imaging.PngBitmapEncoder();
enc.Frames.Add(System.Windows.Media.Imaging.BitmapFrame.Create(bmp));

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