Question

I am trying to write a WPF application where you can draw circles on a window by double clicking it. So far I have this code:

public class ShapeAdorner : Adorner
{
    private readonly Ellipse _circle;

    public ShapeAdorner(UIElement adornedElement, Point circleCenter)
        : base(adornedElement)
    {
        _circle = new Ellipse
        {
            Width = 10,
            Height = 10,
            Stroke = Brushes.Black,
            StrokeThickness = 1.5
        };
        _circle.Margin =
            new Thickness(left: circleCenter.X, top: circleCenter.Y, right: 0, bottom: 0);
        base.AddVisualChild(_circle);
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        _circle.Arrange(new Rect(finalSize));
        return finalSize;
    }

    protected override Size MeasureOverride(Size constraint)
    {
        _circle.Measure(constraint);
        return constraint;
    }

    protected override Visual GetVisualChild(int index)
    {
        return _circle;
    }

    protected override int VisualChildrenCount
    {
        get { return 1; }
    }
}

Here’s the client code:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void Window_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(myLabel);
        adornerLayer.Add(new ShapeAdorner(adornedElement: myLabel, circleCenter: e.GetPosition(myLabel)));
    } 
}

The circles are supposed to be centered at the point where you double click the window; however, the circles drawn by the code above are centered below and to the right of “the double click point”. How can this be fixed?

EDIT: myLabel has Height=350 and Width=525. Let’s say that I double click the point (X,Y); then the circle gets plotted at ((350+X)/2,(525+Y)/2).

EDIT 2: Just for completeness, here’s the .xaml file:

<Window x:Class="Adorners.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Adorners project" Height="350" Width="525" MouseDoubleClick="Window_MouseDoubleClick">
    <Grid>
        <Label Name="myLabel" Content="my label" Background="Red"></Label>
    </Grid>
</Window>

Answer 1

The previous answers are correct. However, the main problem was that I had omitted these two lines:


_circle.HorizontalAlignment = HorizontalAlignment.Left;
_circle.VerticalAlignment = VerticalAlignment.Top;


The default value Stretch caused a huge offset error.