WPF RenderTargetBitmap保存UIElement内容为空问题

对UIElement进行刷新

Size tSize = new Size(tPenNode.Width, tPenNode.Height);
tPenNode.Measure(tSize);
tPenNode.Arrange(new Rect(new Point(0, 0), tSize));

DrawingVisual tDrawingVisual = new DrawingVisual();
            using (DrawingContext context = tDrawingVisual.RenderOpen())
            {
                VisualBrush tVisualBrush = new VisualBrush(tPenNode);
                tVisualBrush.Stretch = Stretch.Fill;
                context.DrawRectangle(tVisualBrush, null, new Rect(0, 0, 80, 80));
                context.Close();
            }

            RenderTargetBitmap tRenderTargetBitmap = new RenderTargetBitmap(80,80, 96, 96, PixelFormats.Default);
            tRenderTargetBitmap.Render(tDrawingVisual);

            using (FileStream tFileStream = new FileStream("C:\\1.png",
                                        FileMode.Create, FileAccess.Write))
            {

                PngBitmapEncoder tPngBitmapEncoder = new PngBitmapEncoder();
                tPngBitmapEncoder.Interlace = PngInterlaceOption.On;
                tPngBitmapEncoder.Frames.Add(BitmapFrame.Create(tRenderTargetBitmap));
                tPngBitmapEncoder.Save(tFileStream);
                tFileStream.Close();
            }