Question

Say that I have a DrawingBrush that has three colors hard-coded, i.e. a border, a foreground, and a background.

<!-- Resource -->
<DrawingBrush x:Key="EventIcon" Stretch="Uniform">
    <DrawingBrush.Drawing>
        <DrawingGroup>
            <DrawingGroup.Children>
                <GeometryDrawing Brush="#FF9200CE" Geometry="F1 M 51.2119,61.4688L 43.4193,61.4688L 43.4194,29.318L 27.8341,29.318L 27.834,61.4688L 20.0414,61.4688L 35.6267,77.1353L 51.2119,61.4688 Z "/>
                <GeometryDrawing Brush="#FFB400FF" Geometry="F1 M 44.4789,64.2014L 40.2667,64.2667L 40.13,29.318L 27.8341,29.318L 27.834,61.4688L 20.0414,61.4688L 33.8667,75.1467L 44.4789,64.2014 Z "/>
                <GeometryDrawing Geometry="F1 M 51.2119,61.4688L 43.4193,61.4688L 43.4194,29.318L 27.8341,29.318L 27.834,61.4688L 20.0414,61.4688L 35.6267,77.1353L 51.2119,61.4688 Z ">
                    <GeometryDrawing.Pen>
                        <Pen Thickness="2" StartLineCap="Round" EndLineCap="Round" LineJoin="Round" Brush="#FF3D0033"/>
                    </GeometryDrawing.Pen>
                </GeometryDrawing>
                <GeometryDrawing Brush="#FFFFFFFF" Geometry="F1 M 33.7559,53.2538L 32.6202,40.9989L 32.6202,35.3362L 37.3531,35.3362L 37.3531,40.9989L 36.2333,53.2538L 33.7559,53.2538 Z M 32.6202,59.6771L 32.6202,54.9442L 37.3531,54.9442L 37.3531,59.6771L 32.6202,59.6771 Z "/>
            </DrawingGroup.Children>
        </DrawingGroup>
    </DrawingBrush.Drawing>
</DrawingBrush>

<!-- Usage -->
<Rectangle Width="16" Height="16" Fill="{StaticResource EventIcon}" />

Question
What would be the best approach to be able to change these colors from the parent Rectangle, yet still have a default fallback?

As I write this question, I have thought to two possible solutions…

Possible Solution #1
Using a RelativeSource binding to connect each to their equivalent property, e.g. {Binding Path=BorderBrush, RelativeSource={RelativeSource AncestorType={x:type Rectangle}} however:

1. Rectangle being a Shape does not have BorderBrush properties;
2. I could not provide a default value. Specifying FallbackValue in the binding won’t work as the binding would resolve and take it’s default value. (Edit: As I write this, I thinking that I could possibly use the NullValue property).

Possible Solution #2
Write an attached property that takes an array of colors/brushes and then have a converter to map it to the GeometryDrawing.Brush. Provide a default value using the Binding.IsNull property as I can guarantee a null value is return if it cannot map if the attached property is null or that color is not remapped.

Answer 1

I would go with a dynamic resource reference, define the defaults at the application level (Application.Resources) and change them locally by adding brushes with the same key in some control’s resources.