如何强制Silverlight容器扩展/收缩到其子控件的大小？

<UserControl SizeToContent="WidthAndHeight">

this.SizeToContent = SizeToContent.WidthAndHeight;

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
       <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
    <!-- 子控件放在这里 -->
</Grid>

<Viewbox Stretch="Uniform">
    <Grid>
        <!-- 子控件放在这里 -->
    </Grid>
</Viewbox>