[C# Markup] TemplateBind to PathFigure.StartPoint and LineSegment.Point crashes

[MartinZikmund]

Discussed in #2405

^{Originally posted by kucint July 2, 2024}
Create a simple Control that uses Path as Template:
Path is just a simple line between two points that are represented by dependency properties: StartHotspot and EndHotspot

public MyControl()
{
    DefaultStyleKey = typeof(MyControl);

    Style = new Style<MyControl>().Setters(s => s.Template(
        ctrl => new Path()
        .Stroke(Colors.Blue)
        .StrokeThickness(3)
        .Data(new PathGeometry().Figures(new PathFigureCollection()
        {
            new PathFigure()
            //.StartPoint(new Point(10, 30))                                // works fine
            .StartPoint(x => x.TemplateBind(() => ctrl.StartHotspot))       // crashes
            .Segments(new PathSegmentCollection()
            {
                new LineSegment()
                //.Point(new Point(80, 50))                                 // works fine
                .Point(x => x.TemplateBind(() => ctrl.EndHotspot))          // crashes
            })
        })))
    );
}

#region StartHotspot
public Point StartHotspot
{
    get { return (Point)GetValue(StartHotspotProperty); }
    set { SetValue(StartHotspotProperty, value); }
}

public static readonly DependencyProperty StartHotspotProperty =
    DependencyProperty.Register(
        nameof(StartHotspot),
        typeof(Point),
        typeof(MyControl),
        new PropertyMetadata(new Point(0, 0)));
#endregion

#region EndHotspot
public Point EndHotspot
{
    get { return (Point)GetValue(EndHotspotProperty); }
    set { SetValue(EndHotspotProperty, value); }
}

public static readonly DependencyProperty EndHotspotProperty =
    DependencyProperty.Register(
        nameof(EndHotspot),
        typeof(Point),
        typeof(MyControl),
        new PropertyMetadata(new Point(0, 0)));
#endregion

The control is now consumed in a frame:

public MainPage() => this
    .DataContext(new BindableMainPageModel(), (page, vm) => page
    .Background(ThemeResource.Get<Brush>("ApplicationPageBackgroundThemeBrush"))
    .Content(new StackPanel()
    .VerticalAlignment(VerticalAlignment.Center)
    .HorizontalAlignment(HorizontalAlignment.Center)
    .Children(
        new TextBlock()
        .Text("Hello Uno Platform!"),

        // implementing Path directly works fine:
        new Path()
        .Stroke(Colors.DarkGreen)
        .StrokeThickness(3)
        .Data(new PathGeometry().Figures(new PathFigureCollection()
        {
            new PathFigure()
            .StartPoint(new Point(10,10))
            .Segments(new PathSegmentCollection() {
                new LineSegment()
                .Point(new Point(80,30))
            })
        })),

        // implementing Path bound to MVU model works fine:
        new Path()
        .Stroke(Colors.HotPink)
        .StrokeThickness(3)
        .Data(new PathGeometry().Figures(new PathFigureCollection()
        {
            new PathFigure()
            .StartPoint(() => vm.StartPoint)
            .Segments(new PathSegmentCollection() {
                new LineSegment()
                .Point(() => vm.EndPoint)
            })
        })),

         // implementing Path inside MyControl bound to control's properties fails:
        new MyControl()
        .StartHotspot(new Point(10, 30))
        .EndHotspot(new Point(80, 50))
    )));

Environment:

  "Uno.Sdk": "5.2.175"

<TargetFrameworks>
  net8.0-windows10.0.19041;
  net8.0-desktop;
</TargetFrameworks>

• under net8.0-desktop; following exception is thrown:
  Exception thrown: 'System.InvalidCastException' in Uno.Extensions.Markup.dll
  Unable to cast object of type 'Microsoft.UI.Xaml.Media.PathFigure' to type 'Microsoft.UI.Xaml.FrameworkElement'.

• under net8.0-windows10.0.19041; following exception is thrown:
  No installed components were detected.
  Cannot apply a Style with TargetType 'Microsoft.UI.Xaml.Controls.ControlTemplate' to an object of type 'UnoPathApp.Controls.MyControl'.
  Note that the following fix has been applied:
  Setting styles in templated control causes control to crash under windows #2316

repro: UnoPathApp.zip

[kazo0]

This should be fixed in the latest -dev packages for markup and will be in the next stable release.

Let us know if you are still having issues after updating!

cc: @MartinZikmund