grid binding to DynamicResource XmlDataProvider is not updating after xml changes

Below code is successfully adding node in testdata.xml after clicking add button, but unable to refresh the tree in grid1. xaml code:

<Window x:Class="XmlDataTree.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="XmlData Tree Test"
        Width="250"
        Height="350" >
    
       <Window.Resources>

        <XmlDataProvider x:Key="xmldata"
                         Source="testdata.xml"
                         XPath="/root" />
        <HierarchicalDataTemplate DataType="Node"
                                  ItemsSource="{Binding XPath=./*}">
            <StackPanel Orientation="Horizontal">
                <TextBlock Margin="0"
                           Text="Node:" />
                <TextBlock Margin="5,0,0,0"
                           Text="{Binding XPath=@name}" />
            </StackPanel>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate DataType="leaf">
            <StackPanel Orientation="Horizontal">
                <TextBlock Margin="0"
                           Text="Leaf:" />
                <TextBlock Margin="5,0,0,0"
                           Text="{Binding XPath=@name}" />
            </StackPanel>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate DataType="root"
                                  ItemsSource="{Binding XPath=./*}">
            <TextBlock Margin="0"
                       Text="ROOT" />
        </HierarchicalDataTemplate>

    </Window.Resources>

  
    <!--
        This sets the DataContext of the Grid to the XmlProvider
        with a key of xmldata (set above in the Window.Resources
        section).
    -->
    <Grid Name="grid0">
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>

        <Grid DataContext="{DynamicResource xmldata}" Grid.Row="0" x:Name="grid1">
            <!--
            Setting the ItemsSource to "{Binding}" causes the TreeView
            to use the Grid's DataContext as its source, and the
            templates describe h        ow each tag is to be displayed and
            handled, in case the node has descendent nodes.
        -->
            <TreeView Name="dirTree"
                  HorizontalAlignment="Stretch"
                  VerticalAlignment="Stretch"
                      ItemsSource="{Binding}"
                  VirtualizingStackPanel.IsVirtualizing="False"
                  VirtualizingStackPanel.VirtualizationMode="Standard" />
        </Grid>
        <Grid Grid.Row="1">
            <Button Name="Add" Grid.Row="1" Width="50" Height="20" Content="Add Button" HorizontalAlignment="Left" Click="AddNode"></Button>
            <TextBox Height="23" Grid.Row="1" HorizontalAlignment="Left"  Name="textBox1"  VerticalAlignment="Top" Width="127" />
  
        </Grid>
    </Grid>
</Window>

cs file code:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Xml;

namespace XmlDataTree
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void xmladdnode()
        {
            string filename = @"testdata.xml";

            //create new instance of XmlDocument
            XmlDocument doc = new XmlDocument();

            //load from file
            //doc.Load(filename);

            //XmlDocument doc3 = new XmlDocument();
            doc.Load(filename);
            XmlNode root1 = doc.DocumentElement;
            //Create a new attrtibute.  
            XmlElement elem = doc.CreateElement("Node");
            XmlAttribute attr = doc.CreateAttribute("name");
            attr.Value = "6";
            elem.Attributes.Append(attr);
            root1.InsertAfter(elem, root1.LastChild);
          
            doc.Save(filename);
            XmlDataProvider oProv = grid1.DataContext as XmlDataProvider;
            oProv.Refresh();

        }

        private void AddNode(object sender, RoutedEventArgs e)
        {
            Button clicked = (Button)sender;
            MessageBox.Show("Button's name is: " + clicked.Name);
            xmladdnode();
         
        }

    }
}