Silverlight ListBox ItemTemplate:选择项时使某些元素可见

<ListBox ItemsSource="{Binding Path=Programs}" >                        
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel>
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="{Binding Begin, Converter={StaticResource TimeOfDayConverter}}" Margin="0,0,10,0" Width="46" />
                    <TextBlock Text="{Binding Title}" FontSize="30" />
                </StackPanel>
               <TextBlock x:Name="txtDescription" Text="{Binding Description}" Margin="56,0" Visibility="Collapsed" />
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>

我需要把文本块命名为 txtDescription 默认情况下是折叠的,但在选择项目时将其设置为可见。我该怎么做(最好是在XAML中)?