方案： 使用Rectangle填充，Rectangle的属性设置如下：HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" Stroke="#FF008BFF"
Demo：
<Border Height="480" Width="640" BorderBrush="#FFFF0000" BorderThickness="2,2,2,2">
<Grid x:Name="LayoutRoot">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*"/>
<ColumnDefinition Width="1*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
</Grid.RowDefinitions>
<Rectangle HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" Stroke="#FF008BFF"/>
<Rectangle HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" Stroke="#FF008BFF" Grid.Column="1"/>
<Rectangle HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" Stroke="#FF008BFF" Grid.Row="1"/>
<Rectangle HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" Stroke="#FF008BFF" Grid.Row="1" Grid.Column="1"/>
</Grid>
</Border>