C# WPF - Rounded background color

Question

How can i make the red corner rounded ? I want to set the CornerRadius only of MainBorder and not for the first and last InnerBorder

If I set the background of textBlocks (instead of InnerBorders) the behavior is the same

<Border Name="MainBorder" Background="Transparent" Width="250" Height="250"  BorderBrush="Black" BorderThickness="3" CornerRadius="20" Margin="500,500,0,0">
            <Grid>                                                
                <Grid.RowDefinitions>                               
                    <RowDefinition Height="1*"/>                 
                    <RowDefinition Height="1*"/>
                    <RowDefinition Height="1*"/>
                </Grid.RowDefinitions>
                <Border Name="brdInner1" Grid.Row="0" BorderBrush="Black" BorderThickness="0,0,0,0" Background="Transparent">
                    <TextBlock />
                </Border>
                    <Border Name="brdInner2" Grid.Row="1" BorderBrush="Black" BorderThickness="0,3,0,0" Background="Red">
                    <TextBlock />
                </Border>
                    <Border Name="brdInner3" Grid.Row="2" BorderBrush="Black" BorderThickness="0,3,0,0" Background="Red">
                    <TextBlock />
                </Border>
            </Grid>
            </Border>

Answer 1

Use the ClippingBorder class from here and just change the type of your main border:

<local:ClippingBorder x:Name="MainBorder" ...>