Silverlight伸展子元素填充父容器？

<UserControl x:Class="SilverlightApplication1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.ColumnDefinitions>
           <ColumnDefinition Width="*" />
           <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

       <Border Grid.Column="0" Grid.Row="0" Background="Red" />
       <Border Grid.Column="1" Grid.Row="0" Background="Blue" />
       <Border Grid.Column="0" Grid.Row="1" Background="Green" />
       <Border Grid.Column="1" Grid.Row="1" Background="Yellow" />
    </Grid>
</UserControl>