[C#/WPF] BlockUIContainer 엘리먼트 사용하기

<FlowDocument>
    ...
    <BlockUIContainer>
        <Polygon
            Points="144 48, 200 222, 53 114, 235 114, 88 222"
            Stroke="Blue"
            StrokeThickness="5">
            <Polygon.Fill>
                <LinearGradientBrush StartPoint="0 0" EndPoint="1 0">
                    <GradientStopCollection>
                        <GradientStop Offset="0"   Color="Red"   />
                        <GradientStop Offset="0.5" Color="Green" />
                        <GradientStop Offset="1"   Color="BLue"  />
                    </GradientStopCollection>
                </LinearGradientBrush>
            </Polygon.Fill>
        </Polygon>
    </BlockUIContainer>
    ...
</FlowDocument>

※ BlockUIContainer는 UIElement 타입을 포함할 수 있다.