The link is dead.

These articles are great.

I realized what it exactly meant after some testing the next day. And I did go down the ItemContainerStyle route (which at the moment is working all right).

The problem is that if you have Path/Line objects mixed with other UIElements , you cannot apply generic Canvas.Left/Canvas.Right logic. I ended up deriving from ListBox and ListBoxItem to handle this. It works but is not pretty.

Thanks again though. I would have never got the hang of this had it not been for your excellent post.

Cheers,
Nishant

If you are using a data template, then the item being added to the Items collection is *not* a UI element. In your example, it is your TestItemVM object.

If you create an observable collection of UIElement objects, then those objects will become direct children of the items panel… they will *not* be wrapped in a ContentPresenter. But, in my experience, there is rarely a good reason to maintain your own collection of UIElements. It is much better to go with VM objects.

You can still use a Canvas as the ItemsPanel and use a binding on your ItemContainerStyle to set the appropriate Canvas.Left, Canvas.Top, etc, properties.

Hope this makes sense!

Cheers,
-dw

Been reading up your posts for a while now and must mention that all your posts on WPF are exceptional.

Have a question with this one.

“* If a UIElement is added to the Items collection of an explicit ItemsControl instance (as opposed to an instance of a derived class like ListBox), it will become a direct child of the items panel. If a non-UIElement is added, it will be wrapped within a ContentPresenter.”

Does this also apply to UIElements instantiated with datatemplates and bound to the ItemsSource? I was under the impression thats the case but my test shows otherwise and it sucks because I thought it would give a ready to use ItemsSource property on a canvas.

class TestItemVM : DependencyObject
{
string _text = “test”;
public string Text
{
get { return _text; }
set { _text = value; }
}
}

class TestVM : DependencyObject
{
public ObservableCollection Items
{
get
{
ObservableCollection items = new ObservableCollection();
items.Add(new TestItemVM());
return items ;
}
}
}

It’s hard to say why its not working without seeing a repro. It probably has to do with the data against which you are binding. If its XML data, then try simply including a ContentPresenter without setting the Content property (the framework does funny stuff when binding against an xml source). Otherwise, you will need to debug the binding to see what is happening.

If you have a simple repro, you can send it to me at ask[at]drwpf[dot]com.

Cheers,
-dw

The scenario described in that article arises because of a circular rooted reference. This can happen, for example, when a child of a panel contains a binding whose source object is its parent and whose source property is the Children collection of that parent. The problem is that the source property contains a rooted reference back to the child. Normally, binding to the Children collection of a panel would not be a problem, but doing so from one of the actual children will create a problem, as described in the KB article.

The best advice I can give on avoiding that scenario is to bind only to your view model objects, wherever possible. Then the potential for this type of leak won’t even arise. Typically, the only time I use bindings to source objects that are not in the view model is when I’m creating a control style or template. For anything else, I will modify my view model to expose the required properties.

Hope this helps!
-dw