Question

I’ve seen this question asked numerous times and have gone over my code over and over. But when I assign to my SelectedItem-bound property with an object, it does not update the display’s selected item. It appears that the ListBox thinks the object I assign is not a member of its items.

public class MainViewModel : ViewModelBase
{
    //...
    public SortedObservableCollection<TubeViewModel> Items { get; private set; }
    public TubeViewModel SelectedTube { //get, set, propertychanged, etc. }
}

<ListBox x:Name="TubeList"
         Margin="10"
         ItemsSource="{Binding Items}"
         ItemTemplate="{StaticResource TubeTemplate}"
         SelectedItem="{Binding SelectedTube, Mode=TwoWay}" 
         SelectionMode="Single"
         VirtualizingStackPanel.IsVirtualizing="False">
</ListBox>

Here’s the impl from one of the places where I try to set SelectedTube – definitely occurs on the main thread.

 var match =
    from t in Items
    where t.Model.DataFileName == filename
    select t;
 if (match.Any())
    SelectedTube = match.First();

I was noticing that the SelectedTube was never highlighted unless I manually clicked on it, but kind of ignored it. But then I wanted to ScrollIntoViewCentered the selected item, so I added a DependencyProperty in my view to watch for SelectedItem changes. The handler initially looked like so:

private void OnSelectedItemChanged(DependencyObject target, DependencyPropertyChangedEventArgs e)
{
    if (TubeList.SelectedItem == null)
       return;

    TubeList.ScrollIntoViewCentered(TubeList.SelectedItem);
}

But didn’t work if I set the SelectedItem through my binding. After deciding I wanted to only scroll it if it was not on screen, I changed the code to this:

private void OnSelectedItemChanged(DependencyObject target, DependencyPropertyChangedEventArgs e)
{
    if (TubeList.SelectedItem == null)
       return;

    var container = TubeList.ItemContainerGenerator.ContainerFromItem(TubeList.SelectedItem) as FrameworkElement;

    if(!container.IsVisible)
       TubeList.ScrollIntoViewCentered(TubeList.SelectedItem);
}

If SelectedItem is externally set, container is null. Every time. I even added code to check for null and then iterate over the ItemsSource looking for a filename match, then update SelectedItem to make sure SelectedItem was definitely an object in the listbox. No luck.

It’s not a visibility thing, if the item is on screen, it still doesn’t highlight. Do I need to implement some sort of equality function? I’ve wasted more time than one might think possible on this.

thanks.

Answer 1

Maybe this is also related to the fact that ItemContainerGenerators are generated in a separate thread… (at least in the TreeView)

Try calling UpdateLayout() before you ask the ItemContainerGenerators property.