WPF ItemsControl bound to UserControls

Question

An ItemsControl binding to a collection of UserControl objects works fine. However, I would like to apply additional XAML, such as a Border , etc.

However, instead of a Border with the UserControl, only the UserControl itself is rendered. The <ItemsControl.ItemTemplate> does not seem to have any effect.

Question: How can I design an ItemTemplate with additional XAML? Currently, this tag seems to be "ignored".

---

ViewModel: ObservableCollection<UserControl> MyUserControls

<ItemsControl ItemsSource="{Binding MyUserControls, lementName=popupContainer}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <Grid />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <Border ...>
                <ContentControl Content="{Binding}" />
            </Border>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

Answer 1

A look at the reference source reveals that the IsItemItsOwnContainerOverride method of the ItemsControl class has this implementation:

protected virtual bool IsItemItsOwnContainerOverride(object item)
{
    return (item is UIElement);
}

So if you pass a collection of UIElements to the ItemsSource of an ItemsControl, these elements are used directly as item containers, without the usual wrapping inside a ContentPresenter . Hence no ItemTemplate is applied at all.

So the answer to the question

How can I design an ItemTemplate with additional XAML?

is: Not at all if the ItemsSource is a collection of UIElements.

You should instead follow the basic idea of the ItemsControl class, and assign a collection of data item objects to the ItemsSource property. Then select appropriate UI controls by DataTemplates that have their DataType property set to the types of the different data items.

---

Or you create a derived ItemsControl which overrides the IsItemItsOwnContainerOverride method:

public class MyItemsControl : ItemsControl
{
    protected override bool IsItemItsOwnContainerOverride(object item)
    {
        return false;
    }
}