依赖属性非常重要的特点-属性值沿UI元素向下传递

依赖属性的重要特点是当没有拥有该依赖属性的控件指定该属性时候，它会把所在容器的该依赖属性借过来使用

<Window x:Class="WpfApp10.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp10"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    
    <Grid DataContext="6">
        <Grid>
            <Grid>
                <Grid>
                    <StackPanel>
                        <Button Content="btn" Click="Button_Click"></Button>

                    </StackPanel>
                </Grid>
            </Grid>
        </Grid>
    </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 WpfApp10
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Button button = (Button)sender;
            MessageBox.Show(button.DataContext.ToString());
        }
    }
}