Question

I have a ListView bound to a view model which contains an ObservableCollection in a simple example app.

public class ViewModel : INotifyPropertyChanged
{
    public ViewModel(){}

    public event PropertyChangedEventHandler PropertyChanged;

    public ObservableCollection<Item> _items;
    public ObservableCollection<Item> Items 
    {
        get 
        { 
            return this._items; 
        }
        set
        {
            if (value != this._items)
            {
                this._items = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("Items"));
            }
        }         
    }
}
public class Item
{
    public string Name;
}

The following function is bound to the SelectionChanged event of the ListView

private void ItemListView_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    model.Items.Add(new Item { Name = "added item" });
    model.Items = new ObservableCollection<Item> { new Item { Name = "new item 1" }};
}

When the event fires, this should happen

1. new item (“added item”) appended to existing ObservableCollection
2. ObservableCollection set to a new collection [single item, “new item 1”]

What actually happens:

1. ObservableCollection set to new collection [single item, “new item 1”]
2. new item (“added item”) appended to end of new collection

Can anyone explain why these are happening in the wrong order?

Answer 1

Can anyone explain why these are happening in the wrong order?

My guess is that they’re not happening in the wrong (reversed) order but the append is happening twice. Executing

 model.Items = ... ;

in the SelectionChanged of those same Items is pretty bold. It will trigger a SelectionChanged again, and only because the Selection then remains at none (Index -1) you do not get into an infinite loop.