Invisible pixels are just as clickable as real pixels! [Tip: Use a Transparent brush to make "empty" parts of a XAML element respond to mouse and touch input]

Tip

Use a Transparent brush to make "empty" parts of a XAML element respond to mouse and touch input

Explanation

I got a question yesterday and thought the answer would make a good addition to my Development Tips series. As you probably know, WPF, Silverlight, and Windows Phone support a rich, hierarchical way of laying out an application's UI. Elements can be created in XAML or in code and respond to input by firing the relevant events (MouseLeftButtonDown, Click, etc.). Input events bubble from the element "closest" to the user all the way up to the root element (stopping if an event is marked Handled). Every now and then someone finds that an element they expect to be getting input is not (and they've made sure none of its children are "eating" the event). The most common reason is that the element doesn't have any pixels for the user to click on! For example, in a 100x100 panel containing a short message, only the text pixels are considered part of the panel and respond to mouse input - everything else passes "through" the empty area and bubbles up to the parent. This behavior enables the creation of elements with any shape, but sometimes it's not what you want. Fortunately, it's simple to get empty parts of an element to respond to input: just draw some pixels! And while a Brush of any color will do the trick, painting with Transparent pixels is a fantastic way to keep empty space looking empty while also being clickable!

Good Example

<Grid
    Background="Transparent"
    MouseLeftButtonDown="Grid_MouseLeftButtonDown">
    <TextBlock
        Text="You can click anywhere in the Grid!"
        HorizontalAlignment="Center"
        VerticalAlignment="Center"/>
</Grid>

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