Howcome the get/set in dependency property doesn't do anything?

Question

I've created a dependency property like this:

public partial class MyControl: UserControl
{
   //...

   public static DependencyProperty XyzProperty = DependencyProperty.Register("Xyz",typeof (string),typeof (MyControl),new PropertyMetadata(default(string)));

   public string Xyz
   {
       get { return (string) GetValue(XyzProperty ); }
       set { SetValue(XyzProperty , value); }            
   }

   //...
}

Then bind it to my wpf window and everything worked fine.

When I tried to add some logic to the setter I notice it wasn't being called. I modify the get;Set up to a point now they look like this:

 get{return null;}
 set{}

And it is still works! How come? What's the use of that GetValue/SetValue calls?

See also [here](http://msdn.microsoft.com/en-us/library/bb613563.aspx#implications) for why you should not put any additional logic into the property wrappers. — Clemens, Aug 10 '12 at 18:15

score 9 · Accepted Answer · answered Aug 10 '12 at 17:26

9

The WPF data binding infrastructure uses the DependencyProperty directly, the Xyz property is a convenience interface for the programmer.

Take a look at the PropertyMetadata in the call to DependencyProperty.Register, you can supply a callback that will run when the property value is changed, this is where you can apply your business logic.

answered Aug 10 '12 at 17:26

Chris Taylor

52,623
10
78
89

I see. But I still need it right? I tried to delete it from source and obviously the compiler complained – OscarRyz Aug 10 '12 at 17:29
1

You only need the property if you are referencing it in your code. Otherwise it is not needed, if you are binding declaratively you will use the name 'Xyz' but will resolve to your dependency property without needing the member property. – Chris Taylor Aug 10 '12 at 17:43

BTownTKD · Answer 2 · 2012-08-10T17:36:41.213

1

The DependencyProperty is the backing store for the XyzProperty. If you access the property through the DependencyProperty interface, it completely bypasses the Property's Get/Set accessor.

Think of it this way:

private int _myValue = 0;

public int MyValue
{
    get { return _myValue; }
    set { _myValue = value; }
}

In this instance, if I manually assign _myValue = 12, obviously the "Set" accessor for the MyValue property won't be called; I completely bypassed it! The same is true for DependencyProperties. WPF's binding system uses the DependencyProperty interfaces directly.

edited Aug 10 '12 at 17:36

answered Aug 10 '12 at 17:29

BTownTKD

7,911
2
31
47

That's why I have to call GetValue/SetValue right? In case I directly invoke `this.Xyz="newValue";` – OscarRyz Aug 10 '12 at 17:32
Right. The Get/Set property wrapper is convenient for you, but ultimately irrelevant to WPF's binding system. – BTownTKD Aug 10 '12 at 17:33

Howcome the get/set in dependency property doesn't do anything?

2 Answers2

Linked