A possible way to see both your "Stringa" and "Sprite" items under the "Progetto" is to make them extend a common class . For example your Progretto can have an ObservableCollection<ProgretoItem> ProgretoItems and this collection will be an union of your collections Stringhe and Sprites (Stringa and Sprite will both extend ProgretoItem). Then you can bind the ItemsSource of the HierarchicalDataTemplate to the ProgretoItems collection.
This will allow you to even define two different DataTemplates for the Stringa and Sprite data items and display different properties. In order to implement this, you will need to define a DataTemplateSelector class and set it as an ItemTemplateSelector in the HierarchicalDataTemplate.
Also you can set an ItemContainerStyleSelector - you will be able to define custom styles for the different ProgretoItem types.
I prepared a sample project for you illustrating such scenario. Please take a look at it and tell me if it satisfies your needs. I would be be glad if I can further assist you.
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