User Controls and Dependency Properties - Pg. 10
       by kirupa  |  19 November 2007

In the previous page, you saw an explanation of the code that went into making our RectangleColor dependency property.

Why Dependency Properties are Useful
The last item this tutorial will discuss is why dependency properties are useful. You can easily get by without ever using dependency properties. If I am trying to convince you of why you need dependency properties, I am obviously not helping by showing you several pages of bizarre and code and explanations.

Joking aside, dependency properties are useful because many styling, data binding, animation, etc. scenarios are made much easier because of them - especially if you are working directly in XAML. To see this in action, let's look at how we can animate our RectangleColor property.

Animating our Background Color
In Blend, go back to Window1.xaml where your InfoRectangle instances are. Select a InfoRectangle instance and give it a snazzy new background color such as what I had in the previous page:

[ an InfoRectangle instance with a modified RectangleColor ]

Now, with this same InfoRectangle selected, let's create a simple animation where you animate your background gradient. Because this tutorial is not about how to create animations, I'm going to breeze through this section. Feel free to read my earlier Introduction to Blend: Animation article for more in-depth coverage.

Create a new storyboard, and at the 0, 1, and 2 second marks, insert a new keyframe. Your timeline should look like the following image:

[ insert a keyframe at times 0, 1, and 2 ]

Now, move your playhead to the 1 second mark. Once there, modify your InfoRectangle's RectangleColor property. For example, I changed my gradient color to be more yellow-ish:

[ at the 1 second mark, I change my RectangleColor property to what you see above ]

If you slide your playhead between the first and second frames, you will see your gradient color animating also! This is something that would not have been able to do as easily had your RectangleColor property not been a dependency property. Sure, you could have gotten away with creating this animation in C# code, but it is much easier to use Blend to visually create your animation as opposed to writing code.

Wrapping Up
The main takeaway is that dependency properties are a link to the internals of WPF's property system. In most cases, you will probably never have to write your own dependency property. All of WPF's controls have majority of their properties implemented as dependency properties, so you gain all of the advantages of using WPF's property system without having to write a single line of code.

There will be situations, though, were you will want to create your own controls such as the InfoRectangle user control you created in this tutorial. By registering some of your newly created/modified properties in your user control, you make it easier for any parent that hosts your user control to have the same level of flexibility normally enjoyed by the controls that come built-in with WPF.

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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