How Dependency Property tell the object to apply to?

Question

(I am totally new to this concept so I may be asking very basic questions.)

A dependency property is registered with the code below:

public static DependencyProperty Register(string name, Type propertyType, Type ownerType, PropertyMetadata typeMetadata);

Logically, it did nothing but associate a property name with the owner type.

So if I have multiple instances of the owner type, and each instance set the DP to different values.

How could these values be stored?

Update 1 - 10:04 AM 10/30/2013

I read about the Attached Property from here: http://wpftutorial.net/DependencyProperties.html

Attached Properties

Attached properties are a special kind of DependencyProperties. They allow you to attach a value to an object that does not know anything about this value.

A good example for this concept are layout panels. Each layout panel needs different data to align its child elements. The Canvas needs Top and Left, The DockPanel needs Dock, etc. Since you can write your own layout panel, the list is infinite. So you see, it's not possible to have all those properties on all WPF controls.

The solution are attached properties. They are defined by the control that needs the data from another control in a specific context. For example an element that is aligned by a parent layout panel.

So in the following code snippet:

<Canvas>
    <Button Canvas.Top="20" Canvas.Left="20" Content="Click me!"/>
    <Button Canvas.Top="40" Canvas.Left="20" Content="Click me!"/>
</Canvas>

Apparently we cannot give all the align properties such as Top, Left to Button. So Canvas defines such properties and they are "attached" to Button control.

When Canvas.Top is specified as an "attribute" of the Button in XAML, it will invoke the SetTop() method which is defined in the Canvas type. And the Button is passed in as the element argument. I think that's how Canvas knows which Button use which Top value.

public static void SetTop(UIElement element, double length);

But I don't see why the Attached Property has to be a Dependency Property? What's the connection between them?

Thanks!

Answer 1

Usually when we define a DependencyProperty, we also define a CLR 'wrapper' that enables us to use the DependencyProperty in code:

public static readonly DependencyProperty ItemsProperty = DependencyProperty.Register(
    "Items", typeof(ObservableCollection<string>), typeof(MainWindow), 
     new UIPropertyMetadata(new ObservableCollection<string>()));

public ObservableCollection<string> Items
{
    get { return (ObservableCollection<string>)GetValue(ItemsProperty); }
    set { SetValue(ItemsProperty, value); }
}

Here you can see the GetValue and SetValue methods that @Clemens was talking about. We get access to these methods in a Window and/or UserControl because they both extend the DependencyObject class. You can also see that the Items property here is not static... it is just the definition of the DependencyProperty that is static.

---

UPDATE >>>

There's not really much point in asking why does an Attached Property have to be a DependencyProperty? because in .NET, they just are... they were just designed like that. A better question might be, what benefit does an Attached Property get from being a DependencyProperty?

The answer to that would be the same as if asked what benefit does a property get from being a DependencyProperty? The main benefits being that these properties can be used in Bindings, Styles, Animations and Resources among other things. More details can be found from the (already linked to in the comments) two very important pages on MSDN for any WPF developers:

Dependency Properties Overview

Attached Properties Overview