Question

I am currently working on a sort of image processing tool that will contain certain selection tools.
In order to show the current selection to the user (for example a circle) three events are used (all inside a certain canvas):

public MainWindow()
{        
    InitializeComponent();
}

private Ellipse _ellip = new Ellipse();
private Point _p = new Point();
private bool _mouse = false;
private double _xcor, _ycor;

private void mouseDown(object sender, MouseButtonEventArgs e)
{
    _p = Mouse.GetPosition(canvas1);
    _xcor = _p.X;
    _ycor = _p.Y;
    _mouse = true;
}

private void newPos(object sender, MouseEventArgs e)
{

    if (mouse)
    {
        _ellip.Visibility = Visibility.Visible;
        _p = Mouse.GetPosition(canvas1);
        _ellip.Margin = new Thickness((_xcor + _p.X) / 2 - Math.Sqrt((_xcor - _p.X) * (_xcor - _p.X) + (_ycor - _p.Y) * (_ycor - _p.Y)) / 2, (_ycor + _p.Y) / 2 - Math.Sqrt((_xcor - _p.X) * (_xcor - _p.X) + (_ycor - _p.Y) * (_ycor - _p.Y)) / 2, 0, 0);
        ellip.Height = Math.Sqrt((_xcor - _p.X) * (_xcor - _p.X) + (_ycor - _p.Y) * (_ycor - _p.Y));
        ellip.Width = Math.Sqrt((_xcor - _p.X) * (_xcor - _p.X) + (_ycor - _p.Y) * (_ycor - _p.Y));
    }

    InvalidateVisual();
}

private void mouseUp(object sender, MouseButtonEventArgs e)
{
    mouse = false;
    ellip.Visibility = Visibility.Hidden;
}

This works all fine under one condition: There is some kind of shape behind the circle. Somehow the events MouseLeftButtonDown/Up and MouseMove are not raised if used on empty area in the canvas1. Once I e.g. create a rectangle filling out the canvas, everything works just fine. But that brings with it another problem: If the rectangle is bigger than the canvas it basically goes out of the canvas and the events also work outside of the canvas (as long it is on top of the rectangle).
I really don’t understand what is going wrong here.

Answer 1

Mouse button actions are recognized by WPF only if hit testing indicates that there is “something” under the mouse pointer. Therefore, you should implement HitTestCore(PointHitTestParameters) on your canvas.

Here is an explanation how to override default hit testing: http://msdn.microsoft.com/en-us/library/ms752097.aspx#overriding_default_hit_testing .