Question

In WPF, what, really, does it mean to be a “dependency property”?

I read Microsoft’s Dependency Properties Overview, but it’s not really sinking in for me. In part that article says:

Styles and templates are two of the chief motivating scenarios for using dependency properties. Styles are particularly useful for setting properties that define application user interface (UI). Styles are typically defined as resources in XAML. Styles interact with the property system because they typically contain “setters” for particular properties, as well as “triggers” that change a property value based on the real-time value for another property.

And then the example code is this:

<Style x:Key="GreenButtonStyle">
  <Setter Property="Control.Background" Value="Green"/>
</Style>
....
<Button Style="{StaticResource GreenButtonStyle}">I am green!</Button>

But I’m not getting what is special about this. Does it just imply, that when I set Style on the button to the given style, that I am actually setting Background implicitly? Is that the crux of it?

Answer 1

In WPF, what, really, does it mean to be a “dependency property”?

In order to be a dependency property, the property must actually be defined as a DependencyProperty, statically, on the class. The dependency property system is very different than a standard CLR property.

Dependency properties are handled very differently, though. A type defines a dependency property statically, and provides a default value. The runtime actually doesn’t generate a value for an instance until it’s needed. This provides one benefit – the property doesn’t exist until requested for a type, so you can have a large number of properties without overhead.

This is what makes the styling work property, but is also important to allow attached properties, property “inheritance” through the visual tree, and many other things WPF relies on.

For example, take the DataContext dependency property. Typically, you set the DataContext dependency property for a Window or a UserControl. All of the controls within that Window, by default, “inherit” their parent’s DataContext proeprty automatically, which allows you to specify data bindings for controls. With a standard CLR property, you’d need to define that DataContext for every control in the window, just to get binding to work properly.