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?
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.