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 :

 <ComboBox ItemsSource="{Binding SelectedItemsSource, Mode=OneWay}" 
           x:Name="c1" 
           IsSynchronizedWithCurrentItem="True">                                    
       <i:Interaction.Triggers>
          <ei:PropertyChangedTrigger Binding="{Binding ElementName=c1,Path=ItemsSource}">
              <local:RetriveLastSelectedIndexChangePropertyAction                   
                      PropertyName="SelectedIndex" 
                      Value="{Binding LastSelectedIndex}" 
                      TargetName="c1"/>
          </ei:PropertyChangedTrigger>                        
       </i:Interaction.Triggers>
  </ComboBox>            

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

Answer 2

Try changing your binding from:

<ei:PropertyChangedTrigger Binding="{Binding ItemsSource,RelativeSource={RelativeSource Self}}">

to

<ei:PropertyChangedTrigger Binding="{Binding ItemsSource,ElementName=c1}">

Answer 3

Try setting property IsSynchronizedWithCurrentItem to true for your combobox and remove the trigger completely -

<ComboBox ItemsSource="{Binding SelectedItemsSource, Mode=OneWay}"
          IsSynchronizedWithCurrentItem="True">             
</ComboBox>