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。