Which geometry does FormattedText.BuildGeometry return?

Question

It returns a Geometry, but that is an abstract class. I am creating a text shape and want to cache this geometry.
That BuildGeometry could returns different types, feels suspect and therefore I may be doing something wrong. I could break the code (will do as soon as it runs) but how can I be sure it will be of the same type always?

public class Label : ShapeBase
    {
        RectangleGeometry geometry = new RectangleGeometry();

        protected override Geometry DefiningGeometry
        {
            get { return geometry; }
        }

        protected override Size MeasureOverride(Size constraint)
        {
            return constraint;
        }

        public string Text
        {
            get { return (string)GetValue(TextProperty); }
            set { SetValue(TextProperty, value); }
        }

        public static readonly DependencyProperty TextProperty =
            DependencyProperty.Register("Text", typeof(string), typeof(Label), new UIPropertyMetadata(string.Empty, OnTextChanged));

        private static void OnTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            Label label = (Label)d;
            label.SetGeometry(label.Text);
            label.InvalidateVisual();
        }

        private void SetGeometry(string text)
        {
            FormattedText formattedtext = new FormattedText(text, CultureInfo.CurrentCulture, FlowDirection.LeftToRight, new Typeface("Tahoma"), 16, Brushes.Black);
            GeometryGroup group = (GeometryGroup)formattedtext.BuildGeometry(new Point(0, 0));
        }
    }

Answer 1

If you set a breakpoint at formattedtext.BuildGeometry and inspect the returned value, you'll realize that it is a GeometryGroup:

var geometry = formattedText.BuildGeometry(new Point());
var geometryGroup = geometry as GeometryGroup;

if (geometryGroup != null)
{
    foreach (var childGeometry in geometryGroup.Children)
    {
        // do something with the child geometries...
    }
}

When I test this with a simple FormattedText, the children of the top-level GeometryGroup are themselves GeometryGroups, with PathGeometries as their children. My assumption is that the second-level GeometryGroups each contain one text row of the FormattedText object, whereas the PathGeometries contain single characters or glyphs.

Answer 2

From your code it looks like a RectangleGeometry.

    RectangleGeometry geometry = new RectangleGeometry();

    protected override Geometry DefiningGeometry
    {
        get { return geometry; }
    }