Proper use of PropertyChangedTrigger and ChangePropertyAction

Question

I\'m trying to set a default selected value when ItemsSource property changes on my ComboBox

My xaml :

Answer 1

has in addition to the current answers which led me on the way to my solution what i actually needed to accomplish is retrieving the last selected item when switching between itemsSources (plural)

from an article i found : "for each ItemsSource binding, a unique CollectionView is generated.."

i concurred that as long as the view exists each binding would generate its own CollectionView and thus hold a reference to CurrentItem and CurrentPosition if decorated with

IsSynchronizedWithCurrentItem="True"

so i created my own ChangePropertyAction Class :

 public class RetriveLastSelectedIndexChangePropertyAction : ChangePropertyAction
 {                
    public int LastSelectedIndex
    {
        get { return (int)GetValue(LastSelectedIndexProperty); }
        set { SetValue(LastSelectedIndexProperty, value); }
    }

    public static readonly DependencyProperty LastSelectedIndexProperty =
        DependencyProperty.Register("LastSelectedIndex", typeof(int), typeof(RetriveLastSelectedIndexChangePropertyAction), new UIPropertyMetadata(-1));

    protected override void Invoke(object parameter)
    {
        var comboBox = this.AssociatedObject as ComboBox;
        this.SetValue(LastSelectedIndexProperty, comboBox.Items.CurrentPosition);            
    }        
 }

and invoked it using PropertyChangedTrigger as follows :

hope this helps if any one needing to retrieve their last selected item with out any messy code in their DataContext , enjoy.