Use UIElement as Clip in WPF - wpf

Question

Please pardon my ignorance- I'm very new to WPF.
I am looking to implement a minor, visual effect in my application that gives the look of "inner" rounded corners. The window in question has a dark border that encapsulates several UIElements, one of which is a StatusBar, located at the bottom of the window. This StatusBar has a dark background that matches the window's border. Above the StatusBar is a content view, which is currently a Grid- its background is semi-transparent (I think that this is something of a constraint- you can see through the content view to the desktop below). I would like for the content view (represented by the transparent, inner area in the figure below) to have the look of rounded corners, though I expect to have to sort of create the illusion myself.
(Can't post the image because I'm a lurker and not a poster- please find the drawing here)
My first approach was to add a Rectangle (filled with the same, dark color as the border) immediately above the StatusBar and to assign a Border with rounded corners to its OpacityMask (similar to the solution proposed by Chris Cavanagh**). Sadly, the effect that is produced is the exact opposite of that which I am trying to achieve.
I understand that the Clip property can be of use in this sort of situation, but it seems to me that using any sort of Geometry will prove to be inadequate as it won't be dynamically sized to the region in which it resides.
EDIT: Including my XAML:
<Grid Background="{StaticResource ClientBg}" Tag="{Binding OverlayVisible}" Style="{StaticResource mainGridStyle}">
<DockPanel LastChildFill="True">
<!-- Translates to a StackPanel with a Menu and a Button -->
<local:FileMenuView DockPanel.Dock="Top" />
<!-- Translates to a StatusBar -->
<local:StatusView DockPanel.Dock="Bottom" />
<!-- Translates to a Grid -->
<local:ContentView />
</DockPanel>
</Grid>
Any pointers are more than welcome- I'm ready to provide more indepth detail if necessary.
** http://www.dotnetkicks.com/wpf/WPF_easy_rounded_corners_for_anything

Answer 1

EDIT: Now I got what you mean. In fact you can use Path + OpacityMask approach. You have to draw "inverted" path, to use it as opacity mask. But I have simpler and faster solution for you :). Use Border + CornerRadius, and fill the gaps with solid paths. Just try the following code in Kaxaml and let me know if this is what you were looking for:
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Width="240"
Height="320"
AllowsTransparency="True"
Background="Transparent"
WindowStyle="None">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="24"/>
<RowDefinition Height="*"/>
<RowDefinition Height="24"/>
</Grid.RowDefinitions>
<Border Background="Black"/>
<Border Grid.Row="1" BorderBrush="Black" BorderThickness="5">
<Grid>
<Border Background="White" CornerRadius="0, 0, 5, 5" Opacity="0.7"/>
<Path
Width="15"
Height="15"
HorizontalAlignment="Left"
VerticalAlignment="Bottom"
Data="M10,10 L5,10 L5,5 C4.999,8.343 6.656,10 10,10 z"
Fill="Black"
Stretch="Fill"/>
<Path
Width="15"
Height="15"
HorizontalAlignment="Right"
VerticalAlignment="Bottom"
Data="M10,10 L5,10 L5,5 C4.999,8.343 6.656,10 10,10 z"
Fill="Black"
Stretch="Fill">
<Path.RenderTransform>
<TransformGroup>
<ScaleTransform ScaleX="-1"/>
<TranslateTransform X="15"/>
</TransformGroup>
</Path.RenderTransform>
</Path>
</Grid>
</Border>
<Border Grid.Row="2" Background="Black"/>
</Grid>
</Window>
PS: You can simplify this solution by avoiding render transforms, but you got the idea.