Property Value Inheritance and More WPF Concepts
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In this second part of a four-part series on important concepts in the Windows Presentation Foundation, you'll learn about property value inheritance, attached properties, and more. This article is excerpted from chapter three of the book
Windows Presentation Foundation Unleashed, written by Adam Nathan (Sam's Publishing; ISBN: 0672328917).
Property Value Inheritance
The term property value inheritance (or property inheritance for short) doesn’t refer to traditional object oriented class-based inheritance, but rather the flowing of property values down the element tree. A simple example of this can be seen in Listing 3.4, which updates the Window from Listing 3.1 by explicitly setting its FontSize and FontStyle dependency properties. Figure 3.4 shows the result of this change. (Notice that the Window automatically resizes to fit all the content thanks to its slick SizeToContent setting!.)
Figure 3.4. The About dialog with FontSize and FontStyle set on the root Window.
LISTING 3.4 The About Dialog with Font Proper ties Set on the Root Window
<Window xmlns="http://schemas.microsoft.com/winfx/ 2006/xaml/presentation"
Title="About WPF Unleashed" SizeToContent="WidthAndHeight"
FontSize="30" FontStyle="Italic"
Background="OrangeRed">
<StackPanel>
<Label FontWeight="Bold" FontSize="20" Foreground="White">
WPF Unleashed (Version 3.0)
</Label>
<Label>© 2006 SAMS Publishing</Label>
<Label>Installed Chapters:</Label>
<ListBox>
<ListBoxItem>Chapter 1</ListBoxItem>
<ListBoxItem>Chapter 2</ListBoxItem>
</ListBox>
<StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
<Button MinWidth="75" Margin="10">Help</Button>
<Button MinWidth="75" Margin="10">OK</Button>
</StackPanel>
<StatusBar>You have successfully registered this product.</StatusBar>
</StackPanel>
</Widow>
For the most part, these two settings flow all the way down the tree and are inherited by children. This affects even the Buttons and ListBoxItems, which are three levels down the logical tree. The first Label’s FontSize does not change because it is explicitly marked with a FontSize of 20, overriding the inherited value of 30. The inherited FontStyle setting of Italic affects all Labels, ListBoxItems, and Buttons, however, because none of them have this set explicitly.
Notice that the text in the StatusBar is unaffected by either of these values, despite the fact that it supports these two properties just like the other controls. The behavior of property value inheritance can be subtle in cases like this for two reasons:
- Not every dependency property participates in property value inheritance. (Internally, dependency properties can opt in to inheritance by passing FrameworkPropertyMetadataOptions.Inherits to DependencyProperty.Register.)
- There may be other higher-priority sources setting the property value, as explained in the next section.
In this case, the latter reason is to blame. A few controls such as StatusBar, Menu, and ToolTip internally set their font properties to match current system settings. This way, users get the familiar experience of controlling their font via Control Panel. The result can be confusing, however, because such controls end up “swallowing” any inheritance from proceeding further down the element tree. For example, if you add a Button as a logical child of the StatusBar in Listing 3.4, its FontSize and FontStyle would be the default values of 12 and Normal, respectively, unlike the other Buttons outside of the StatusBar.
DIGGING DEEPER
Property Value Inheritance in Additional Places
Property value inheritance was originally designed to operate on the element tree, but it has been extended to work in a few other contexts as well. For example, values can be passed down to certain elements that look like children in the XML sense (because of XAML’s property element syntax) but are not children in terms of the logical or visual trees. These pseudochildren can be an element’s triggers or the value of any property (not just Content or Children) as long as it is an object deriving from Freezable. This may sound arbitrary and isn’t well documented, but the intention is that several XAML-based scenarios "just work" as you would expect without having to think about it.
Next: Support for Multiple Providers >>
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