WPF Shape bounding rectangle is always empty - wpf

Question

I have the following code in my WPF application:
public MainWindow()
{
InitializeComponent();
p1 = new Point(50, 50);
p2 = new Point(355, 50);
p3 = new Point(50, 355);
p4 = new Point(355, 355);
Loaded += (x, y) => Draw();
//ContentRendered += (x, y) => Draw();
}
I create a Polygon shape and try to get its bounding rectangle several ways:
private void Draw()
{
Polygon polygon = new Polygon();
polygon.Stroke = System.Windows.Media.Brushes.White;
polygon.Points.Add(p1);
polygon.Points.Add(p2);
polygon.Points.Add(p3);
polygon.Points.Add(p4);
canvas.Children.Add(polygon);
boundingRect = polygon.TransformToVisual(canvas).TransformBounds(new Rect(polygon.RenderSize));
boundingRect = polygon.TransformToVisual(polygon).TransformBounds(System.Windows.Controls.Primitives.LayoutInformation.GetLayoutSlot(polygon));
boundingRect = polygon.RenderTransform.TransformBounds(new Rect(polygon.RenderSize));
boundingRect = GetRectOfObject(polygon);
boundingRect = polygon.RenderedGeometry.Bounds;
}
private Rect GetRectOfObject(FrameworkElement _element)
{
Rect rectangleBounds = new Rect();
rectangleBounds = _element.RenderTransform.TransformBounds(new Rect(0, 0, _element.ActualWidth, _element.ActualHeight));
return rectangleBounds;
}
However, I always get {0;0;0;0}.
On debug, I can see the points but its size is rendered 0:
How should I calculate the correct way?

Answer 1

According to the MSDN remarks of the Measure method, calling UpdateLayout will obtain the necessary data (though it should be treated carefully), so this way the measuring works:
private void Draw()
{
Polygon polygon = new Polygon();
polygon.Stroke = System.Windows.Media.Brushes.White;
polygon.Points.Add(p1);
polygon.Points.Add(p2);
polygon.Points.Add(p3);
polygon.Points.Add(p4);
canvas.Children.Add(polygon);
polygon.UpdateLayout();
boundingRect = polygon.TransformToVisual(canvas).TransformBounds(new Rect(polygon.RenderSize));
}
Thanks #ChrisF for the hints.