How can I draw a border with squared corners in wpf?

Question

You know, like Battlestar paper! I have given this a few goes but now I'm stumped. I haven't yet gone down the geometery route, so I'll explain this as best as I can.

I'd like the border to be sizable, but contain fixed-size corners, just like CornerRadius does. Instead of rounded corners, I'd like them to be tapered, like:

/---------\
|         |
|         |
\_________/

I've done two attempts at this:

1. My first attempt attempts to manipulate a border class. This just doesn't work, as stretching the shape ruins the geometry and scale.
2. The second attempt was a bit more out the box. Literally. I created a 3x3 grid and filled it with 4 borders, each with a thickness of 2,0,0,0 - 0,2,0,0 - 0,0,2,0 and 0,0,0,2 respectively. The final step, is the join the borders up with a Line. Here where my question lies....

First attempt

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"&gt;
   <Grid>
      <Grid.Resources>
         <Style x:Key="MyPoly" TargetType="Polygon">
            <Setter Property="Points">
               <Setter.Value>
                  <PointCollection>
                     <Point X="0.10" Y="0.01"/>
                     <Point X="0.50" Y="0.01"/>
                     <Point X="0.60" Y="0.10"/>
                     <Point X="0.60" Y="0.50"/>
                     <Point X="0.50" Y="0.60"/>
                     <Point X="0.10" Y="0.60"/>
                     <Point X="0.01" Y="0.50"/>
                     <Point X="0.01" Y="0.10"/>
                  </PointCollection>
               </Setter.Value>
            </Setter>
         </Style>
      </Grid.Resources>
      <Border
         Width="100"
         Height="100"
         BorderBrush="Black"
         BorderThickness="3"
         CornerRadius="5"/>
      <Grid Width="400"
            Height="300">
         <Polygon
            Stroke="Purple"
            StrokeThickness="2"
            Style="{StaticResource MyPoly}" Stretch="Fill">
            <Polygon.Fill>
               <SolidColorBrush Color="Blue" Opacity="0.4"/>
            </Polygon.Fill>
            <Polygon.LayoutTransform>
               <ScaleTransform ScaleX="1" ScaleY="1"/>
            </Polygon.LayoutTransform>
         </Polygon>
      </Grid>
   </Grid>
</Page>

Second attempt

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" SnapsToDevicePixels="True">
    <Grid>
        <Grid.Resources>
        </Grid.Resources>
        <Grid Width="200" Height="350" SnapsToDevicePixels="True">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="10"/>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="10"/>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="10"/>
                <RowDefinition Height="*"/>
                <RowDefinition Height="10"/>
            </Grid.RowDefinitions>
            <Border Grid.Column="0" Grid.Row="1" Margin="0" BorderBrush="Red" BorderThickness="2,0,0,0" Padding="0" SnapsToDevicePixels="True"/>
            <Border BorderThickness="1" BorderBrush="Black">
                <Line SnapsToDevicePixels="True" Stretch="Fill" Stroke="Red" StrokeThickness="2" X1="0" X2="1" Y1="1" Y2="0">
                </Line>
            </Border>
            <Border Grid.Column="1" Grid.Row="0" BorderBrush="Red" BorderThickness="0,2,0,0" SnapsToDevicePixels="True"/>
            <Border Grid.Column="2" Grid.Row="1" BorderBrush="Red" BorderThickness="0,0,2,0" SnapsToDevicePixels="True"/>
            <Border Grid.Column="1" Grid.Row="2" BorderBrush="Red" BorderThickness="0,0,0,2" SnapsToDevicePixels="True"/>
        </Grid>
    </Grid>
</Page>

The Line is set to scale to the grid size. Setting the Line properties to X1="0" X2="1" Y1="1" Y2="0" and using Stretch="Fill" expands the Line to the edges. However, it ends up looking like this:

(Annoyingly, I can't post images, I need to go answer someone elses questions to earn some rep. So instead please go to this link to see the line, or paste the above XAML into Kaxaml.) http://img375.imageshack.us/img375/1996/border1.png

I drew a magenta border around the Grid element hosting the Line, to make the problem more obvious.

How can I expand the line to really fill the gap (for example by inflating the drawable area within the grid), or, is there a better way?

Also, transformations distort the line, making it thicker. I tried scaling up but there wasn't a consistency to this. Endcaps on the line look just as bad (Triangle for example).

Finally, this method is still flawed, because I want to be able to set the corner size in the future, so having the edge width for the row/column set to 10 seems like a stumbling point. Binding to a property might solve that, I've never done that in a Style though.

Thanks for reading, Tom

Answer 1

The WPF border is inheriting from class Decorator. It is pretty easy to write your own Decorator. Below one draws a border around a child with "tucked in" corners.

class FunkyBorder : Decorator
{
    public Brush BorderBrush
    {
        get { return (Brush)GetValue(BorderBrushProperty); }
        set { SetValue(BorderBrushProperty, value); }
    }

    public static readonly DependencyProperty BorderBrushProperty =
        DependencyProperty.Register("BorderBrush", 
                                    typeof(Brush), 
                                    typeof(FunkyBorder), 
                                    new UIPropertyMetadata(Brushes.Transparent));

    protected override void OnRender(DrawingContext drawingContext)
    {
        // TODO, make pen thickness and corner width (currently 10) into dependency properties.
        // Also, handle case when border don't fit into given space without overlapping.

        if (_pen.Brush != BorderBrush)
        {
            _pen.Brush = BorderBrush;
        }

        drawingContext.DrawLine(_pen, new Point(0, 10), new Point(10, 0));
        drawingContext.DrawLine(_pen, new Point(10, 0), new Point(ActualWidth - 10, 0));
        drawingContext.DrawLine(_pen, new Point(ActualWidth - 10, 0), new Point(ActualWidth, 10));
        drawingContext.DrawLine(_pen, new Point(0, 10), new Point(0, ActualHeight - 10));
        drawingContext.DrawLine(_pen, new Point(ActualWidth, 10), new Point(ActualWidth, ActualHeight - 10));
        drawingContext.DrawLine(_pen, new Point(0, ActualHeight - 10), new Point(10, ActualHeight));
        drawingContext.DrawLine(_pen, new Point(10, ActualHeight), new Point(ActualWidth - 10, ActualHeight));
        drawingContext.DrawLine(_pen, new Point(ActualWidth - 10, ActualHeight), new Point(ActualWidth, ActualHeight - 10));
    }

    private Pen _pen = new Pen(Brushes.Transparent, 2);
}

Use like this:

   <BorderTest:FunkyBorder BorderBrush="Red">
        <TextBlock Text="Hello" />
    </BorderTest:FunkyBorder>

Answer 2

To avoid the nasty breaks at the end, you could use a Polygon or PolyLine:

    <Polygon
        Stroke="Red"
        StrokeThickness="2"
        Points="
            0,1 1,0
            1,0 20,0
            20,0 21,1
            21,1 21,20
            21,20 20,21
            20,21 1,21
            1,21 0,20
            0,1 1,0
        "
        Stretch="Fill"
        />

The width I picked is arbitrary...