DependencyProperty in wpf

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what are the advantages of Dependency Property. where it is used in wpf? sample code?

Posted 9-Oct-12 21:29pm

akchandra9

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3 solutions

Solution 3

This is from http://stackoverflow.com/questions/2505234/need-a-short-and-clear-definition-for-dependency-properties[^]:

A DependencyProperty is a property whose value depends (or can depend) on some other source (such as animation, data binding, styles, or visual tree inheritance). A regular property's value is stored in the object it belongs to, while you can think of a dependency property as being stored in a database somewhere. This database is essentially composed of a dictionary that maps (object, property) pairs to their values, along with a mapping of which properties depend on other properties (e.g. so when you change the DataContext of a Panel, it can notify all the children inside the panel).

So why do they store property values in some magic database somewhere? There are a few reasons:

•It reduces storage space. Adding a property (even if its value is null) to a class adds 4 bytes (8 for a 64-bit process) of space to every instance of the class. A DependencyProperty only takes up space when an instance has a value. For example, a FrameworkElement has dozens of dependency properties, most of which are never assigned values. If all those properties were stored in the class, each instance would be hundreds of bytes. Instead each instance is only about 40 bytes.

•It enables attached properties. Properties like Canvas.Left and Grid.Row have to be stored on objects that have never heard of a Canvas or Grid, so where do you put them? You put them in a database somewhere.

•It enables automatic property changes. Imagine how you would implement something like styles or property inheritance (the ability to set something like a font or data context on a parent element and have its value propagate to all child elements). Having all of this stored in a database makes it so the code is all in one place instead of being implemented separately for each object and property that needs it.

I am not sure I totally agree with the answer, but I know mostly it is right.

Posted 10-Oct-12 15:54pm

Clifford Nelson

Solution 2

SQL

Each WPF control registers a set of DependencyProperties to the static DependencyProperty class. Each of them consists of a key - that must be unique per type - and a metadata that contain callbacks and a default value.

All types that want to use DependencyProperties must derive from DependencyObject. This baseclass defines a key, value dictionary that contains local values of dependency properties. The key of an entry is the key defined with the dependency property.

When you access a dependency property over its .NET property wrapper, it internally calls GetValue(DependencyProperty) to access the value.
If a local value is available, it reads it directly from the dictionary. If no value is set if goes up the logical tree and searches for an inherited value. If no value is found it takes the default value defined in the property metadata. This sequence is a bit simplified, but it shows the main concept.

Posted 9-Oct-12 23:52pm

akchandra9

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Santhosh Kumar Jayaraman · Accepted Answer · 2012-10-09T21:35:00

SQL

The purpose of dependency properties is to provide a way to compute the value of a property based on the value of other inputs. These other inputs might include system properties such as themes and user preference, just-in-time property determination mechanisms such as data binding and animations/storyboards, multiple-use templates such as resources and styles, or values known through parent-child relationships with other elements in the element tree.

So they're like normal properties, but their value can be automatically computed from the values of other properties.


Check this for sample code

http://www.switchonthecode.com/tutorials/wpf-tutorial-introduction-to-dependency-properties
http://www.betterthaneveryone.com/archive/2010/01/24/wpf-silverlight-xaml-and-dependency-properties.aspx