添加虚线外框

  <Border BorderThickness="{Binding ElementName=mediaItemList,Path=HasItems,
                        Converter={StaticResource booleanToThicknessConverter}}">
                    <Border.BorderBrush>
                        <DrawingBrush Viewport="0,0,2,2" ViewportUnits="Absolute" TileMode="Tile">
                            <DrawingBrush.Drawing>
                                <DrawingGroup>
                                    <GeometryDrawing Brush="Black">
                                        <GeometryDrawing.Geometry>
                                            <GeometryGroup>
                                                <RectangleGeometry Rect="0,0,50,50" />
                                                <RectangleGeometry Rect="50,50,50,50" />
                                            </GeometryGroup>
                                        </GeometryDrawing.Geometry>
                                    </GeometryDrawing>
                                </DrawingGroup>
                            </DrawingBrush.Drawing>
                        </DrawingBrush>
                    </Border.BorderBrush>

  ----other control content---------

  </Border>