WPF ComboBox event for ItemSource changed

Question

I have a particular collection which derives from ObservableCollection and this type of collection I am binding to ItemSource of ComboBox. The problem is that when the ItemSource is changed the SelectedItem is ruined so I need to hook up the event of ItemSource changed so I can manually set the SelectedItem property.
I have tried to use IsSynchronizedWithCurrentItem="True" but that results in selecting the first item from the list, which in my case is not what I want because the SelectedItem is not first in the list. I have created a Behavior and attached to ComboBox but I can not find an event to hook up the ItemSource changes, also I can not rely on property changes of ObservableCollection cause in most cases the Collection has changes on the items in the list and there are no new items added or removed. I will post below my implementation.

<ComboBox ItemsSource="{Binding DisplayZoneNumberSource}"
  SelectedItem="{Binding DisplayZoneNumberSelect, UpdateSourceTrigger=PropertyChanged}"
  ItemTemplateSelector="{StaticResource ComboBoxSourceTemplateSelector}"
  VerticalContentAlignment="Center">
    <i:Interaction.Behaviors>
        <local:ComboBoxItemSourceChangedBehavior/>
    </i:Interaction.Behaviors>
</ComboBox>

public class ComboBoxItemSourceChangedBehavior : Behavior<ComboBox>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.SelectionChanged += AssociatedObject_SelectionChanged;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.SelectionChanged -= AssociatedObject_SelectionChanged;
    }

    private static void AssociatedObject_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        //Implementation
    }
}

"when the ``ItemSource`` is changed the ``SelectedItem`` is ruined" -> what do you mean? — Corentin Pane, Nov 21 '19 at 08:02
https://stackoverflow.com/questions/5242275/combobox-itemssource-changed-selecteditem-is-ruined — Gabriel Costin, Nov 21 '19 at 08:09
Why not subscribing to `DisplayZoneNumberSource.CollectionChanged`? — Corentin Pane, Nov 21 '19 at 08:14
Already said in my post that `CollectionChanged` it is NOT raising when you change a property of an item of the collection — Gabriel Costin, Nov 21 '19 at 08:15
Okay so your question is actually about catching PropertyChanged events of all items in a collection, right? — Corentin Pane, Nov 21 '19 at 08:16
Not exactly as this involves many changes and I want to find a quick workaround. Everytime when user enters in drop-down then ItemSource is updated with a new collection ( It might be same collection but sometimes it is a collection with new items/ removed items or modified items ) . Best solution for me would be to find an event that is raised when ItemSource is changed and I would manually select the SelectedItem — Gabriel Costin, Nov 21 '19 at 08:19
https://learn.microsoft.com/en-us/previous-versions/dotnet/netframework-4.0/ms745795(v=vs.100) and https://stackoverflow.com/questions/4764916/listen-to-changes-of-dependency-property — Lennart, Nov 21 '19 at 08:38
Why do you update with new collection every time? Use the newly "typed in" item and add it to collection. — XAMlMAX, Nov 21 '19 at 14:06

score 1 · Accepted Answer · answered Nov 21 '19 at 14:43

If you create a custom class that inherits from ComboBox, there is a protected OnItemsSourceChanged method that you can override:

public class MyComboBox : ComboBox
{
    protected override void OnItemsSourceChanged(IEnumerable oldValue, IEnumerable newValue)
    {
        base.OnItemsSourceChanged(oldValue, newValue);
        //...
    }
}

Obviously this method won't get called when a property of an individual item in the source collection is set. If you need to this, you need to hook up an event handler for the PropertyChanged of each item whenever an item is added to or removed from the source collection. And this require your type of items to implement INotifyPropertyChanged or some other interface that has a similar event.

Is there any event like `OnItemsSourceChanging` ? Because I can not use the one in the answer because it is too late and `SelectedItem` is already null — Gabriel Costin, Nov 27 '19 at 16:34

WPF ComboBox event for ItemSource changed

1 Answers1