Silverlight 中 自定义的 ListBoxItem如何自适应ListBox的宽度

<ListBox x:Name="xxx">
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
</Style>
</ListBox.ItemContainerStyle>
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel>
<Grid>
<TextBlock Text="hello" HorizontalAlignment="Left"/>
<TextBlock Text="world" HorizontalAlignment="Right"/>
</Grid>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>