Question

Given this XAML:

<Style TargetType="PasswordBox">
    <Setter Property="Background">
        <Setter.Value>
            <VisualBrush TileMode="Tile"
                         Viewport="0,0,10,10" ViewportUnits="Absolute">
                <VisualBrush.Visual>
                    <Canvas Background="{x:Static SystemColors.WindowBrush}">
                        <Path Data="M0,0 L10,10 M0,10 L10,0">
                            <Path.Stroke>
                                <SolidColorBrush Color="{x:Static SystemColors.HighlightColor}"/>
                            </Path.Stroke>
                        </Path>
                    </Canvas>
                </VisualBrush.Visual>
            </VisualBrush>
        </Setter.Value>
    </Setter>
    ...

The canvas background is being ignored, and instead the path is visible over a background that’s transparent to the form behind the PasswordBox. So where should I be setting the “background for the background”?

Answer 1

The problem is that the Canvas has no size.

Change it to this and you should see it:

<Canvas Background="{x:Static SystemColors.WindowBrush}"
        Width="10"
        Height="10">

To reduce the number of references to those dimensions, you can declare them as resources.
Since you are dealing with squares, you could reduce it to one value:

    <Grid.Resources>
        <System:Double x:Key="Width">10</System:Double>
        <System:Double x:Key="Height">10</System:Double>
        <Style TargetType="PasswordBox">
            <Setter Property="Background">
                <Setter.Value>
                    <VisualBrush TileMode="Tile"
                                 ViewportUnits="Absolute">
                        <VisualBrush.Viewport>
                            <Rect Width="{StaticResource Width}"
                                  Height="{StaticResource Height}" />
                        </VisualBrush.Viewport>
                        <VisualBrush.Visual>
                            <Canvas Background="{x:Static SystemColors.WindowBrush}" 
                                    Width="{StaticResource Width}"
                                    Height="{StaticResource Height}" >

Of course, if you are binding to a view-model, you could also drive the dimensions through binding.