wpf 绑定表达式和手动更新源

<Window x:Class="BindingExpressionExam.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">
    <Grid>
        <Slider Name="slider" Minimum="1" Maximum="100" TickFrequency="1" HorizontalAlignment="Left" Margin="118,58,0,0" VerticalAlignment="Top" Width="265"/>


        <!--绑定延时-->
        <!--<TextBox Name="txt" HorizontalAlignment="Left" Height="23" Margin="118,156,0,0" TextWrapping="Wrap" Text="{Binding ElementName=slider, Path=Value,UpdateSourceTrigger=PropertyChanged, Delay=1000}" VerticalAlignment="Top" Width="120"/>-->
        <TextBox Name="txt" HorizontalAlignment="Left" Height="23" Margin="118,156,0,0" TextWrapping="Wrap" Text="{Binding ElementName=slider, Path=Value,UpdateSourceTrigger=Explicit}" VerticalAlignment="Top" Width="120"/>

        <Button Content="Button" HorizontalAlignment="Left" Margin="326,156,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click_1"/>

    </Grid>

</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;


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


        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            BindingExpression be= txt.GetBindingExpression(TextBox.TextProperty);
            be.UpdateSource();
        }
    }
}