WPF user control - ItemsControl not applied

Question

I am creating a WPF user control that comes from TabControl. In the ControlTemplate, I use ItemControl to display the list associated with the template (an observable collection of type FileMenuItem). While the program is running, I get the following error in the output window:

ItemTemplate and ItemTemplateSelector are ignored for items that are already Container Type ItemsControl; Type = 'FileMenuItem'

The FileMenuItem type is inferred from MenuItem. If I changed the base class to DependencyObject, the code really runs and the template is applied (so there is an option). I googled a bug and couldn't find anything about this, does anyone encounter this while developing custom controls? Despite the fact that I have a workaround, I would like to understand what is happening, and I think that using MenuItem as a base class is a cleaner implementation.

I can post more code if this helps. Thanks!

Answer 1

The purpose of a DataTemplate (e.g., ItemTemplate) is to provide a visualization for a data object. In particular, it defines a set of elements to add to the visual tree instead of the data provided by ContentPresenter or ItemsPresenter. In your case, your source list is a set of objects that can already be added directly to the visual tree for display in the user interface.

You can see this in the following simplified example, where only “Three” is displayed in “Red” because the first two elements are defined in a form that can be displayed directly by the ComboBox.

<ComboBox> <ComboBox.ItemTemplate> <DataTemplate> <TextBlock Text="{Binding}" Foreground="Red"/> </DataTemplate> </ComboBox.ItemTemplate> <ComboBoxItem>One</ComboBoxItem> <ComboBoxItem>Two</ComboBoxItem> <sys:String>Three</sys:String> </ComboBox>