Combining LineGeometry with EllipseGeometry (in code, not XAML)

Question

I'm trying to create a custom shape with WPF. For starters I was just trying to create a simple line, that has a circle at each end (I know there are LineCaps, but that's not what I'm looking for).

I've looked into some tutorials and the easiest way to do that, seems to use a CombinedGeometry. However I can't get it to work properly. Here is my code that creates the geometry object:

protected override Geometry DefiningGeometry
    {
        get
        {
            Point ellipseCenter1 = new Point(X1 - this.CapDiameter / 2, Y1 - this.CapDiameter / 2);
            Point ellipseCenter2 = new Point(X2 - this.CapDiameter / 2, Y2 - this.CapDiameter / 2);
            var ellipse1 = new EllipseGeometry(ellipseCenter1, CapDiameter, CapDiameter);
            var ellipse2 = new EllipseGeometry(ellipseCenter2, CapDiameter, CapDiameter);
            var line = new LineGeometry(this.StartPoint, this.EndPoint);

            var combined1 = new CombinedGeometry(GeometryCombineMode.Union, ellipse1, line);
            var combined2 = new CombinedGeometry(GeometryCombineMode.Union, combined1, ellipse2);

            // Freeze the geometry for performance benefits
            combined2.Freeze();

            return combined2;
        }
    }

However, for some reason, this doesn't draw the line. It draws both circles, but not the line. Obviously I'm missing something here, could someone point out what? :)

Just in case it matters, here is the rest of the class:

#region Dependency Properties
public static readonly DependencyProperty X1Property = 
    DependencyProperty.Register(
        "X1", 
        typeof(double),
        typeof(CappedLine), 
        new FrameworkPropertyMetadata(
                0.0, 
                FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure
            )
    );
public static readonly DependencyProperty Y1Property =
    DependencyProperty.Register(
        "Y1",
        typeof(double),
        typeof(CappedLine),
        new FrameworkPropertyMetadata(
                0.0,
                FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure
            )
    );
public static readonly DependencyProperty X2Property =
    DependencyProperty.Register(
        "X2",
        typeof(double),
        typeof(CappedLine),
        new FrameworkPropertyMetadata(
                0.0,
                FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure
            )
    );
public static readonly DependencyProperty Y2Property =
    DependencyProperty.Register(
        "Y2",
        typeof(double),
        typeof(CappedLine),
        new FrameworkPropertyMetadata(
                0.0,
                FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure
            )
    );
public static readonly DependencyProperty CapDiameterProperty =
    DependencyProperty.Register(
        "CapDiameter",
        typeof(double),
        typeof(CappedLine),
        new FrameworkPropertyMetadata(
                0.0,
                FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure
            )
    );

#endregion
#region CLR Properties

public double X1
{
    get { return (double)base.GetValue(X1Property); }
    set { base.SetValue(X1Property, value); }
}

public double Y1
{
    get { return (double)base.GetValue(Y1Property); }
    set { base.SetValue(Y1Property, value); }
}

public double X2
{
    get { return (double)base.GetValue(X2Property); }
    set { base.SetValue(X2Property, value); }
}

public double Y2
{
    get { return (double)base.GetValue(Y2Property); }
    set { base.SetValue(Y2Property, value); }
}

public Point StartPoint
{
    get { return (new Point(X1, Y1)); }
}

public Point EndPoint
{
    get { return (new Point(X2, Y2)); }
}

public double CapDiameter
{
    get { return (double)base.GetValue(CapDiameterProperty); }
    set { base.SetValue(CapDiameterProperty, value); }
}

#endregion

And here is the XAML code I used to test it:

<Window x:Class="HG.WPF.Shapes.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:shapes="clr-namespace:Tomers.WPF.Shapes"

    Title="WPF Custom Shapes" Height="300" Width="300" ResizeMode="NoResize">

    <Canvas Background="Black">
        <shapes:CappedLine X1="30" Y1="30" X2="200" Y2="200" CapDiameter="5" Fill="White" Stroke="White" StrokeThickness="2"></shapes:CappedLine>        
    </Canvas>

</Window>

Answer 1

Your LineGeometry objects will vanish if you try to Union them. From MSDN:

The GeometryCombineMode property specifies how the two geometries will be combined. Note that CombinedGeometry combines the area specified by two geometries, so geometries that do not have area (such as LineGeometry) disappear when combined.

You can use GeometryGroup:

GeometryGroup combined = new GeometryGroup();
combined.Children.Add(ellipse1);
combined.Children.Add(ellipse2);
combined.Children.Add(line);

or you can use CombinedGeometry on the ellipse objects first, and then group that together with the line using GeometryGroup.