listbox绑定XML文件（wpf）

单项的绑定listbox是可以的代码如下
MainWindow.xaml文件
<Window x:Class="ListBoxBing.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <XmlDataProvider x:Key="MyTask" Source="Task.xml"  XPath="/root"/>
        
    </Window.Resources>
    <Grid>
        <ListBox Height="149" HorizontalAlignment="Left" Margin="114,66,0,0" Name="listBox1" VerticalAlignment="Top" Width="205" IsSynchronizedWithCurrentItem="True"
          ItemsSource="{Binding Source={StaticResource MyTask},Mode=OneWay, 
          XPath=Task/@name}" 
       />
        <Button Content="Button" Height="46" HorizontalAlignment="Left" Margin="384,51,0,0" Name="button1" VerticalAlignment="Top" Width="76" Click="button1_Click" />
    </Grid>
</Window>Task.xml文件结构如下<?xml version="1.0" encoding="utf-8"?>
<root>
  <Task Nid="1" name="111"/>
  <Task Nid="2" name="222"/>
  <Task Nid="3" name="333"/>
<root>现在我修改task.xml的内容。界面并没有变化不知道是什么问题，希望知道的哥们指点一下。谢谢