silverlight ListBox 嵌入 textBox 更改内容

 <ListBox Height="100" HorizontalAlignment="Left"  Margin="128,110,0,0" Name="lstBx" VerticalAlignment="Top" Width="120" SelectionChanged="lstBx_SelectionChanged" >
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBox  x:Name="txtTitle" Text="{Binding Title}" Tag="{Binding}" Width="110" TextChanged="TextBox_TextChanged"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>


 private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            TestItem selItem = ((TextBox)sender).Tag as TestItem;
           string txtTitle= ((TextBox)sender).Text;
           if (selItem != null)
            {
                selItem.Title = txtTitle;
                //MessageBox.Show((lstBx.SelectedItem as TestItem).Title.ToString());
              
              
            }
        }