Inverted Corner Radius in WPF (something like a negative Radius)

Question

I try to create a border with a negative Corner radius, like this:

I want the radius on the inside. Is this possible with build in functions or do I have to paint it with a path?

Answer 1

Well I faced the same problem and tried to figure out a solution. Last but not least I came to the conclusion that it is not possible to solve this with a border.

I focussed on drawing my Border with the Path and the CombinedGeometry function.

If anyone is still looking for a solution how to make inverted round corners here is my solution:

<Grid>
            <Path Stroke="Black" Fill="White" StrokeThickness="10" Stretch="Fill">
                <Path.Data>
                    <CombinedGeometry GeometryCombineMode="Exclude">
                        <CombinedGeometry.Geometry1>                                
                            <RectangleGeometry Rect="100,200 200,100"/>
                        </CombinedGeometry.Geometry1>
                        <CombinedGeometry.Geometry2>
                            <EllipseGeometry RadiusX="50" RadiusY="50" Center="300,300" />
                        </CombinedGeometry.Geometry2>
                    </CombinedGeometry>
                </Path.Data>
            </Path>
    <Button Margin="10" Height="10" Width="10"/>
</Grid>

You can resize the Rectanglewith changing the values in RectangleGeometry Rect="100,200 200,100and add inside it any other WPF item.. in this case I added a Button.

Here's the result: