WpF 控件截图

 private void CreateImage()
        {
            FileStream fs = new FileStream("image.png", FileMode.Create);
            RenderTargetBitmap bmp = new RenderTargetBitmap((int)bt.ActualWidth,
                                                           (int)bt.ActualHeight, 100, 100,
                                                          PixelFormats.Pbgra32);
            bmp.Render(this.bt );
            BitmapEncoder encoder = new PngBitmapEncoder();

            encoder.Frames.Add(BitmapFrame.Create(bmp));
            encoder.Save(fs);
            fs.Dispose();
            Close();
            fs.Dispose();


        }