Force transparent background on custom UserControl

Question

I want to create a new UserControl and reroute the Background property to use it elsewhere than in the UserControl.Background property (like it is done on the checkbox for example).

Here is a simple custom usercontrol:

<UserControl xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:Controls"
         xmlns:ed="http://schemas.microsoft.com/expression/2010/drawing" 
         x:Class="Controls.HexagonalTile"
         mc:Ignorable="d" d:DesignWidth="300" d:DesignHeight="300">
<Grid>
    <ed:RegularPolygon Fill="{Binding ElementName=LayoutRoot, Path=Background}" StrokeThickness="5" Stroke="Black"/>
</Grid>

And I want to use it like this:

<Controls:HexagonalTile HorizontalAlignment="Left" Height="100" Width="100" Background="Aqua" />

But when I do this, the corner of my user control, outside of the hexagone, take the background color too. I want them to stay transparent.

Thank's for your help.

Answer 1

The reason why this is happening is because the default ControlTemplate for a UserControl has a Border with a TemplateBinding to the Background property.

However, you can re-template the control like this to achieve your goal:

<UserControl x:Class="WpfApp4.HexagonalTile"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ed="http://schemas.microsoft.com/expression/2010/drawing">
    <UserControl.Template>
        <ControlTemplate TargetType="{x:Type UserControl}">
            <Grid>
                <ContentPresenter />
            </Grid>
        </ControlTemplate>
    </UserControl.Template>
    <Grid>
        <ed:RegularPolygon
            Fill="{Binding Background, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"
            Stroke="Black"
            StrokeThickness="5" />
    </Grid>
</UserControl>

I hope this helps!