WPF DataTemplate内重写BorderBrush，VisualBrush内数据源绑定提示绑定失败

定义DataTemplate 数据模板文件，内容如下

<DataTemplate x:Key="{DataTemplateKey {x:Type VM:TemplateListVM}}" DataType="{x:Type VM:TemplateListVM}">
<Grid Margin="0" Grid.Row="3" Height="50" VerticalAlignment="Top" Visibility="{Binding ShowNoMore, Converter={StaticResource BooleanToVisibilityConverter}}"><Grid.ColumnDefinitions><ColumnDefinition Width="*" /><ColumnDefinition Width="auto" /><ColumnDefinition Width="*" /></Grid.ColumnDefinitions><Border Grid.Column="0" Height="1" Margin="60,0,10,0" BorderThickness="1" Name="LeftBorder"><Border.BorderBrush><VisualBrush><VisualBrush.Visual><Rectangle StrokeDashArray="4 4" Stroke="Gray" StrokeThickness="1" Width="{Binding ElementName=LeftBorder, Path=ActualWidth}" Height="2" /></VisualBrush.Visual></VisualBrush></Border.BorderBrush></Border><TextBlock Grid.Column="1" VerticalAlignment="Center" Text="{x:Static loc:MainWindowLoc.NoMoreData}" Style="{StaticResource BodyStyleWhite}" Foreground="{StaticResource BorderGrayBrush}" /><Border Grid.Column="2" Height="1" Margin="10,0,60,0" BorderThickness="1"><Border.BorderBrush><VisualBrush><VisualBrush.Visual><Rectangle StrokeDashArray="4 4" Stroke="Gray" StrokeThickness="1" Width="{Binding RelativeSource={RelativeSource AncestorType={x:Type Border}}, Path=ActualWidth}" Height="2" /></VisualBrush.Visual></VisualBrush></Border.BorderBrush></Border>       </Grid>
</DataTemplate>

虽然显示效果没有问题，但程序运行时两个Width的绑定均会提示绑定失败：
ActualWidth Rectangle.Width Double 找不到源: RelativeSource FindAncestor, AncestorType=‘System.Windows.Controls.Border’, AncestorLevel=‘1’
或者是找不到LeftBorder。
但如果直接写在userControl里面是没有任何提示的，研究了好久，发现在DataTemplate 需要这样写：

 <Grid Margin="0" Grid.Row="3" Height="50" VerticalAlignment="Top" Visibility="{Binding ShowNoMore, Converter={StaticResource BooleanToVisibilityConverter}}"><Grid.ColumnDefinitions><ColumnDefinition Width="*" /><ColumnDefinition Width="auto" /><ColumnDefinition Width="*" /></Grid.ColumnDefinitions><Grid.Resources><VisualBrush x:Key="leftBorderBrush"><VisualBrush.Visual><Rectangle StrokeDashArray="4 4" Stroke="Gray" StrokeThickness="1" Width="{Binding ElementName=LeftBorder, Path=ActualWidth}"  Height="2" /></VisualBrush.Visual></VisualBrush>  <VisualBrush x:Key="RightBorderBrush"><VisualBrush.Visual><Rectangle StrokeDashArray="4 4" Stroke="Gray" StrokeThickness="1"  Width="{Binding ElementName=RightBorder , Path=ActualWidth}" Height="2" /></VisualBrush.Visual></VisualBrush>            </Grid.Resources><Border Grid.Column="0" Height="1" Margin="60,0,10,0" BorderThickness="1" Name="LeftBorder" BorderBrush="{StaticResource leftBorderBrush}" /><TextBlock Grid.Column="1" VerticalAlignment="Center" Text="{x:Static loc:MainWindowLoc.NoMoreData}" Style="{StaticResource BodyStyleWhite}" Foreground="{StaticResource BorderGrayBrush}" /><Border Grid.Column="2" Height="1" Margin="10,0,60,0" BorderThickness="1" Name="RightBorder"  BorderBrush="{StaticResource RightBorderBrush}"/></Grid>           </Grid>

刚使用wpf做正式项目，暂不清楚两种写法的区别。