WPF: Uninitialized Fill property breaks IsMouseOver

Question

I have this XAML code:

<Window x:Class="TTT_01.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525" MouseLeftButtonDown="Window_MouseLeftButtonDown">
    <Canvas>
        <Rectangle Canvas.Left="146" Canvas.Top="110" Height="100" Name="rectangle1" Stroke="Black" Width="200" />
    </Canvas>
</Window>

and this is the C# code behind:

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

    private void Window_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        if (rectangle1.IsMouseOver)
        {
            MessageBox.Show("Click!");
        }
    }
}

What I see is a white rectangle. When I click into the rectangle I expect the message box to pop up. This doesn't happen though, unless I initialize the Fill property of the rectangle either in XAML or in the MainWindow constructor. Without Fill being initialized rectangle1.IsMouseOver remains false even though I click into the rectangle.

What does Fill have to do with IsMouseOver?

Answer 1

The fill is part of what specifies the hit test area of your controls. In WPF controls generally do a pixel-perfect hit test, not a rectangular one. For example, if you put an <Ellipse/> in you XAML, clicking outside of the circumference, but inside the bounding rectangle won't generate a MouseDown event in the Ellipse (this one is kind of obvious though). In the same way not specifying a Fill for a Shape means that the inside of the shape simply doesn't exist - it hit tests negatively and lets clicks fall through. In other words, shapes without a Fill only constitute of their outline for purposes of hit testing.

If you want to have a border-only transparent rectangle (or any other Shape) that still hit tests on the inside, then specify Fill="Transparent".

This is true not only for shapes, but also for containers like Grid and Border - if you don't specify a Background, then their empty area doesn't register in hit testing.