In ListBox cannot add Items after ItemSource is set to null

Question

I have assigned ItemsSource property of first ListBox 'ListBox1' as the ItemsSource of another ListBox namely 'ListBox2' . If I set ListBox2's ItemsSource as null, then I am unable to add any Items further to ListBox1's ItemsSource.

Below is the xaml snippet,

         <ListBox VerticalAlignment="Top" HorizontalAlignment="Center" Width="150" Margin="0 25 0 0"
                 x:Name="ListBox1" ItemsSource="{Binding Coll,Mode=TwoWay}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding _Name}"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

        <ListBox VerticalAlignment="Top" HorizontalAlignment="Center" Width="150" Margin="0 25 0 0"
                 x:Name="ListBox2" ItemsSource="{Binding Path=ItemsSource,ElementName=ListBox1,Mode=TwoWay}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding _Name}"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

In Code behind, I set the ItemSource of ListBox2 as null on button click like below,

        ListBox2.SetCurrentValue(ListBox.ItemsSourceProperty, null);

Once this is done, I tried to add Items to ListBox1's "Coll" collection, but throws NRE that "Coll" is null.

Any suggestions plz.

Regards, Dinesh Kumar P

Clemens Clemens · Accepted Answer · 2013-08-13T10:33:13

The two-way binding on ListBox2.ItemsSource sets ListBox1.ItemsSource to null when ListBox2.ItemsSource is set to null. Subsequently, the two-way binding on ListBox1.ItemsSource also sets Coll to null.

Don't make the ItemsSource bindings two-way. It is not necessary, and collection changes will still be notified.

<ListBox x:Name="ListBox1"
         ItemsSource="{Binding Coll}" ... >
    ...
</ListBox>
<ListBox x:Name="ListBox2"
         ItemsSource="{Binding Path=ItemsSource, ElementName=ListBox1}" ... >
    ...
</ListBox>

It's not really clear what you are going to achieve, but it may be better to also bind ListBox2.ItemsSource directly to the Coll property.

In ListBox cannot add Items after ItemSource is set to null

1 Answers