How to make the contents of a round-cornered border be also round-cornered?

Question

I have a border element with rounded corners containing a 3x3 grid. The corners of the grid are sticking out of the border. How can I fix that? I tried using ClipToBounds bu

Answer 1

Using @Andrew Mikhailov's solution, you can define a simple class, which makes defining a VisualBrush for each affected element manually unnecessary:

public class ClippedBorder : Border
{
    public ClippedBorder() : base()
    {
        var e = new Border()
        {
            Background = Brushes.Black,
            SnapsToDevicePixels = true,
        };
        e.SetBinding(Border.CornerRadiusProperty, new Binding()
        {
            Mode = BindingMode.OneWay,
            Path = new PropertyPath("CornerRadius"),
            Source = this
        });
        e.SetBinding(Border.HeightProperty, new Binding()
        {
            Mode = BindingMode.OneWay,
            Path = new PropertyPath("ActualHeight"),
            Source = this
        });
        e.SetBinding(Border.WidthProperty, new Binding()
        {
            Mode = BindingMode.OneWay,
            Path = new PropertyPath("ActualWidth"),
            Source = this
        });

        OpacityMask = new VisualBrush(e);
    }
}

To test this, just compile the following two samples:

<!-- You should see a blue rectangle with rounded corners/no red! -->
<Controls:ClippedBorder
    Background="Red"
    CornerRadius="10"
    Height="425"
    HorizontalAlignment="Center"
    VerticalAlignment="Center"
    Width="425">
    <Border Background="Blue">
    </Border>
</Controls:ClippedBorder>

<!-- You should see a blue rectangle with NO rounded corners/still no red! -->
<Border
    Background="Red"
    CornerRadius="10"
    Height="425"
    HorizontalAlignment="Center"
    VerticalAlignment="Center"
    Width="425">
    <Border Background="Blue">
    </Border>
</Border>