Virtualizing Stack Panel Returning Null Selected Item

Question

I was using a stack panel to display listbox items, but when I decided to change it to a virtualizing one the selected item was null sometimes. Here is part of the DataTemplate I was using to invoke the selected item command:

        <i:Interaction.Triggers>
            <ei:DataTrigger Binding="{Binding IsSelected}" Value="True">
                <i:InvokeCommandAction CommandParameter="{Binding}"
                        Command="{Binding DataContext.SelectItemCommand, RelativeSource={RelativeSource AncestorType={x:Type ListBox}, Mode=FindAncestor}}" />
            </ei:DataTrigger>
        </i:Interaction.Triggers>

Here is the ListBox:

<ListBox x:Name="_itemsListBox" 
                         Grid.Row="1"                                                     
                         ScrollViewer.CanContentScroll="true" 
                         ItemsSource="{Binding Items}" 
                         IsSynchronizedWithCurrentItem="True"
                         SelectionMode="Single"                                                   
                         ScrollViewer.VerticalScrollBarVisibility="Visible" 
                         ScrollViewer.HorizontalScrollBarVisibility="Disabled" 
                         ItemTemplate="{StaticResource ListItemTemplate}">

                    <ListBox.ItemContainerStyle>
                        <Style TargetType="{x:Type ListBoxItem}" BasedOn="{StaticResource {x:Type ListBoxItem}}">
                            <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
                        </Style>
                    </ListBox.ItemContainerStyle>                    
                </ListBox>

If I turn off virtualization, this issue doesn't happen. How can I prevent it from returning a null item?

score 0 · Answer 1 · answered Sep 23 '11 at 10:27

0

Maybe you can define a FallBackValue inside your Binding, to get back the FallBackValue instead of null.

answered Sep 23 '11 at 10:27

Jan K.

2,542
2
21
30

score 0 · Answer 2 · edited May 23 '17 at 11:55

0

My best guess would be that your selected item is null when it is out of view in the listbox.

It makes sense I guess because of the virtualization though it is an odd one.

Your solution is likely to be that when your selected item changes you make sure you bring it in to view. Take a look at this question to sort that out.

Good luck & HTH

edited May 23 '17 at 11:55

Community

1
1

answered Sep 30 '11 at 11:20

EightyOne Unite

11,665
14
79
105

Virtualizing Stack Panel Returning Null Selected Item

2 Answers2