Adding VisualChild to Canvas does not work - not displayed - wpf

Question

I have to write a class that inherits from Canvas and that is able to add Visuals to the canvasbase.
So I wrote this code:
class TestCanvas : Canvas
{
VisualCollection visuals;
public TestCanvas()
{
visuals = new VisualCollection(this);
}
protected override void OnMouseDown(MouseButtonEventArgs e)
{
Rectangle rect = new Rectangle
{
Width = 200,
Height = 200,
Stroke = Brushes.Red,
StrokeThickness = 5,
Fill = Brushes.Black
};
visuals.Add(rect);
base.OnMouseDown(e);
}
protected override int VisualChildrenCount
{
get
{
return visuals.Count;
}
}
protected override Visual GetVisualChild(int index)
{
if (index < 0 || index > visuals.Count)
throw new ArgumentOutOfRangeException("index");
return visuals[index];
}
}
But if I click on it and I add this Rectangle it is not displayed.
So does anyone has an idea why this does not work?

Answer 1

If you want to programatically add children to a derived Canvas, you can simply do it like this:
Rectangle rect = new Rectangle
{
Width = 200,
Height = 200,
Stroke = Brushes.Red,
StrokeThickness = 5,
Fill = Brushes.Black
};
Canvas.SetLeft(rect, ...);
Canvas.SetTop(rect, ...);
Children.Add(rect);
No need to go down to the Visual layer and override VisualChildrenCount and GetVisualChild.
If for any other reason you have to use Visuals, then there is no need to use a Canvas. You might derive from UIElement or FrameworkElement.