Problems when adding tabitems dynamically

[hwsoderlund]

Hi, I'm receiving the following error message when I'm trying to add tab items dynamically to a tabcontrol inside a radwindow:  "Element is already the child of another element."
I've uploaded a sample project so you can see what's going on. Basically I'm trying to create a "master" control with the tabcontrol and one initial tabitem. Then I add the other tab items through a dependency property in the child user controls. The problem occurs when you close the first window and then try to open it again. I'm not sure whether this is a problem in your component or if there's something wrong with my own code. I would appreciate it if you could just take a look and see if this problem is something you've come across before.

http://cid-cc43a90a79374ebd.skydrive.live.com/self.aspx/Public/SilverlightApplication10.zip

[Miroslav]

Hello Henrik,

Thank you for your example, this makes debugging a lot easier!

the problem in this case is the way DependencyProperties work - setting a collection as a default value of a dependency property yields unexpected results.

Since a default value (a an object reference will always exist as a value) to the dependency property, this object will be shared, kind of static. So each next instance of the master control will add 2 new TabItems. That can be clearly seen when you debug your project and check the count of the items. With the first instance they are 2, the second 4 and they will get more and more if this could continue.

In essence, the TabControl was trying to add items that were already items of another control.

I would not imagine this is production code, but other good practices include exposing collection properties as interfaces (IList<Somethning>) and using ObservableCollection<Something> (which can be assigned to an IList) when the items are going to be bound as an ItemsSource.

This blog gives more information on the Collection Dependency properties:

http://blogs.telerik.com/ManolDonev/Posts/08-07-25/WPF_The_Static_Nature_of_Dependency_Properties.aspx

To resolve the issue, simply set a local (and therefore unique) value to the property:

TabItems

 

= new List<RadTabItem>();

 

Kind regards,
Miroslav
the Telerik team

Check out Telerik Trainer, the state of the art learning tool for Telerik products.

[hwsoderlund]

That solves my problem. And thanks for the explanation, dependency properties are much clearer to me now.