WPF userControl Render successed,and Append drawing never show on Screen?

Question

this is very simple example to show my question.

1

public class TestControl : UserControl
    {
        public TestControl()
        {
           
        }

        protected override void OnRender(DrawingContext context)
        {
            drawCircle(context);
            base.OnRender(context);
        }

        private void drawCircle(DrawingContext context)
        {
            context.DrawEllipse(Brushes.Red, null, new Point(50, 50), 50, 50);
        }
        public void drawRect()
        {
            DrawingContext context = new DrawingVisual().RenderOpen();
            context.DrawRectangle(Brushes.Green, null, new Rect(0, 0,200, 100));
        }
    }

2、 The Window front

<Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="300"/>
            <RowDefinition Height="50"/>
        </Grid.RowDefinitions>
        <Grid Background="White" x:Name="root">
            <local:TestControl x:Name="progressBar"   HorizontalContentAlignment="Stretch"/>
        </Grid>
        <Grid Grid.Row="1">
            <Button Content="Init" Click="Button_Click" Width="100" Height="40"/>
        </Grid>
        
    </Grid>

and 3、

 public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Loaded += MainWindow_Loaded;
        }

   
        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
         
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            progressBar.drawRect();

        }
    }

Question is:Red Circle in Render() showwed in windows.the green Rect never to show。 dont know why

I think when render finished,the DrawingContext disposed,how to append in the first drawingcontext