Dependency Properties in Silverlight - Page 2
       by kirupa  |  25 February 2009

In the previous page, you received a brief introduction to dependency properties. We ended by starting our deconstruction of the various things that make up a dependency property, more specifically, the register portion of your dependency property identifier. Let's continue from there.

Registering the Dependency Property
Registering your dependency property is handled by DependencyProperty.Register. The Register method takes several arguments to help you customize some of the your dependency property's behavior, so let's look at those arguments now.

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata(""));

The first argument you pass in refers to the name of the DependencyProperty you wish to register. In general, try to keep the name the same as that of your dependency property such as what I've done.

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata(""));

The second argument defines the type of your dependency property. In our case, AuthorName is of type string, so that is the value I pass in to the Register method.

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata(""));

The third argument looks similar to what we wrote earlier. What you are specifying is the type of the class that owns this dependency property. This will vary depending on your scenario, but I have this dependency property declared inside a user control called AuthorTextField. Therefore, AuthorTextField is what I specify here.

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata(""));

The last argument determines the metadata that will be carried along with this dependency property, and that is handled via the PropertyMetadata class. The term metadata often indicates something that isn't always useful, but that may be the wrong impression to get at this point. As you will see very shortly, the arguments you pass in here greatly affect how you may end up using the dependency properties in your application.

Into your PropertyMetadata constructor, you can specify the default value for your dependency property, a method to call when this property's value has changed, or both. In the next section, let's look at that in greater detail.

PropertyMetadata Variants
Like I briefly hinted at in the previous section, you can specify some additional information about your dependency property via the PropertyMetadata object.

Specifying a Default Value
The first variation you can do is change the default value your dependency property has. In my original code, I don't have a default value specified. If you were to examine this dependency property in Expression Blend, this is what you would see...a blank field:

[ no default value results in nothing being displayed ]

Let me specify a default value instead. My new Register code looks as follows with a default value specified:

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata("Sir Arthur Conan Doyle"));

When I rebuild my project and examine this property inside Blend again, notice what I see this time around:

[ your dependency property's default value is displayed ]

Instead of being greeted by a blank value, the default value I specified is what gets displayed instead. In my case, the default value is of type string because that is the type of my dependency property. In whatever you are doing, make sure that your dependency property's default value is the same type as your dependency property. In other words, don't put your value inside quotation marks to signify a string just because I have done so!

Specifying a Callback Method
Another argument you can specify into your PropertyMetadata object is a callback method. This is a method that gets called each time your dependency property's value changes, and the call to this callback method will look as follows:

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata(new PropertyChangedCallback(AuthorNameChanged)));

In the above code, the name of my callback method is called AuthorNameChanged. What I haven't done yet is actually create my AuthorNameChanged method, but that is actually pretty straightforward:

private static void AuthorNameChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)

{

// Code for dealing with author name changes

}

There are a few things that you should note. Because your dependency property identifier (and by extension your Register code) is a static variable, an event handler assigned to it such as AuthorNameChanged must be static as well.

The second thing to note are the two arguments passed into your callback method. The first argument is a reference to your DependencyObject object, and the second argument is a reference to your DependencyPropertyChangedEventArgs.

As long as your callback method has a static modifier and contains two arguments of type DependencyObject and DependencyPropertyChangedEventArgs, you are set! Each time your dependency property's value is changed, your callback method will get called and any code contained in it executed.

Having your Cake and Eating it as Well
The final combination of arguments you can specify for your PropertyMetadata is one where you have both a default value as well as a callback method! This would look as follows:

DependencyProperty.Register("AuthorName", typeof(string), typeof(AuthorTextField), new PropertyMetadata("Sir Arthur Conan Doyle", new PropertyChangedCallback(AuthorNameChanged)));

I specify both my default value as well as create the new PropertyChangedCallback object with my callback method name provided as an argument.

Conclusion
Hopefully this article provided you with a good dose of information on how to use dependency properties in Silverlight 2. While articles like this treat dependency properties as being special, they are actually very common. For further reading on a practical use of dependency properties, I strongly recommend you read my 10 page article on dependency properties and user controls. While that article is WPF based, much of concepts will be similar in Silverlight.

I hope the information helped. If you have any questions or comments, please don't hesitate to post them on the kirupa.com Forums. Just post your question and I, or our friendly forum helpers, will help answer it.

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