Hello Juergen Holzer,
Adding your object to a simple List<> will never reflect in the controls because this class does not support change notification or more concretely, it does not have the ListChanged event. You have to use BindingList<> instead.
In order to get the indices of the selected items you simply need to use the SelectedItems property. It is a collection and contains RadListBoxItem instances. Each RadListBoxItem has an Index property. Keep in mind however that this index is local to the collection, it is not the same as the index of the objects in your binding list. For example if RadListBox is sorted, the indices will not match.
I hope this information is useful.
Please write again if you have other questions.
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