WPF: InvalidateVisual doesn't draw by OnRender.

Question

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.NET4.5

Goal: To use OnRender to draw into ImageBrush by RenderTargetBitmap.
Problem: It doesn't draw.
Question: How to do it using OnRender without adding Control as child to Canvas.Children?
Notes:
1. It works, if to add Control as child to Canvas.Children. You can do by turning the comment off.
2. It works even without calling InvalidateVisual, if to use VisualCollection.
3. The goal is to do it using OnRender without adding Control as child to Canvas.Children.
Code:

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        Canvas canvas = new Canvas();
        this.Content = canvas;
        canvas.Background = Brushes.Black;

        MyImage myimage = new MyImage();
        myimage.InvalidateMeasure();
        myimage.InvalidateArrange();
        myimage.InvalidateVisual();
        myimage.UpdateLayout();

        //canvas.Children.Add(myimage); // Works

        canvas.MouseDown += delegate
        {
            RenderTargetBitmap target = new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Default);
            target.Render(myimage);

            ImageBrush brush = new ImageBrush();
            brush.ImageSource = target;
            canvas.Background = brush;
        };

    }
}

class MyImage : Image
{
    protected override void OnRender(DrawingContext dc)
    {
        base.OnRender(dc);

        dc.DrawRectangle(Brushes.Red, null, new Rect(0, 0, 100, 100));
    }
}

Posted 26-Oct-15 1:45am

Ziya1995

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Ziya1995 · Accepted Answer · 2015-10-28T06:13:00

OnRender doesn't get called until a visual gets added into a visual tree.
If the goal is to get the image without adding the visual to the visual tree, you can use VisualCollection/DrawingVisual.

Code:

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        Canvas canvas = new Canvas();
        this.Content = canvas;
        canvas.Background = Brushes.Black;

        MyImage myimage = new MyImage();
        myimage.InvalidateMeasure();
        myimage.InvalidateArrange();
        myimage.InvalidateVisual();
        myimage.UpdateLayout();

        //canvas.Children.Add(myimage); // You no longer need it

        canvas.MouseDown += delegate
        {
            RenderTargetBitmap target = new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Default);
            target.Render(myimage);

            ImageBrush brush = new ImageBrush();
            brush.ImageSource = target;
            canvas.Background = brush;
        };

    }
}

class MyImage : Image
{
    public MyImage()
    {
        visuals = new VisualCollection(this);
        visual = new DrawingVisual();
        visuals.Add(visual);

        Draw();
    }

    private void Draw()
    {
        using (DrawingContext dc = visual.RenderOpen())
            dc.DrawRectangle(Brushes.Red, null, new Rect(0, 0, 100, 100));
    }

    VisualCollection visuals;
    DrawingVisual visual;

    protected override Visual GetVisualChild(int index)
    {
        return visuals[index];
    }
    protected override int VisualChildrenCount
    {
        get { return visuals.Count; }
    }
}

Source:
https://social.msdn.microsoft.com/Forums/en-US/81ccabb8-4f38-46ba-827c-d0a2b66372de/wpf-can-onrender-render-if-a-control-is-not-added-as-child[^]