WPF ListBox not updating with the ItemsSource

Question

I have what I believe should be simple two-way databinding in WPF setup, but the listbox (target) is not updating as the collection changes.

I'm setting this ItemsSource of the ListBox programmatically:

lstVariable_Selected.ItemsSource = m_VariableList;

And the ListBox is declared as follows:

           <ListBox Margin="5" Name="lstVariable_Selected">
              <ListBox.ItemsPanel>
                 <ItemsPanelTemplate>
                    <VirtualizingStackPanel Orientation="Horizontal"/>
                 </ItemsPanelTemplate>
              </ListBox.ItemsPanel>
              <ListBox.ItemTemplate>
                 <DataTemplate>
                    <Border BorderBrush="Gray" BorderThickness="1" Margin="0">
                       <TextBlock FontSize="25" Text="{Binding Path=Name}" />
                    </Border>
                 </DataTemplate>
              </ListBox.ItemTemplate>
           </ListBox>

When I initially set the ItemsSource, the ListBox (which is not visible at the time) gets its items set. However, if I go view the ListBox, updates seem to stop at that point.

I can then remove an item from the m_VariableList collection, and it does not disappear from the ListBox. Likewise, if I add one, it doesn't appear.

What gives?

Answer 1

Is your m_VariableList implementing INotifyCollectionChanged? If it's not an ObservableCollection, then changes to it's contents will not automatically be reflected in the UI.

Answer 2

The problem is not in the XAML that you have provided. I used the same XAML successfully in a test application; however, I was able to replicate the issue you are experiencing by re-instantiating the m_VariableList variable.

When the m_VariableList is given a new instance, or pointed to a new object, it is not reflected in the ListBox because the control has its own reference to the data. This may not be the cause of your problem, but I'd recommend looking over your code-behind to ensure that the variable is not getting re-instantiated.