title: Combobox ItemsSource Binding (MVVM) - how can I reset the ItemsList
body: <p>I Have a combobox with ItemsSource bound to an ObservableCollection.
  A RadioButton allows the user to select a mode. for each mode, a dedicated differnt list should be available in the combobox.</p>
  
  <p>My Problem with using an Observablecollection is that once the public observableCollection is initialized, modifying it is only by Clear all and Add new list, item by item.</p>
  
  <p>This is highly in-efficient and causes problems with validations i have on the selected item.</p>
  
  <p>Can you please suggest another method which will allow me to reset the items source so that it would be refreshed in the UI without Remove/Add?</p>