I've a combobox(CadastreCombo) and a stackPanel containing a label and another combobox (DesSecCombo).

Try 1 : I try to make the stackPanel visible depending if the SelectedItem of the first combobox(CadastreCombo) is null or not.

I tryed that, but it's not working and I don't understand why it's not working
<StackPanel Name="DesSecStackPanel" 
                        Margin="30,10,0,10" >
                    <Style TargetType="StackPanel">
                        <Setter Property="Visibility" Value="Collapsed" />
                            <DataTrigger Binding="{Binding ElementName=CadastreCombo, Path=IsSelected}"    Value="True">
                                <Setter Property="Visibility"  Value="Visible"/>
                <Label Content="Désignation secondaire : "/>
                <ComboBox Name="DesSecCombo" 
                          ItemsSource="{Binding Path=SelectedCadastre.DesignationsSecondaires}"
                          SelectedItem="{Binding Path=SelectedDesSec}" />

Try 2 : Because the second combobox depends of the selectedItem of the first one, time to time the second combobox containes only one item. In this case (if the comboBox.itemsSource count>0), automatically select the item.

I'm able to solve both problems via the "backstage", but I try to learn and understanding xaml. And it's still complicated for me.

Thanks for help, but especially for explanations.