WPF 寻找控件模板中的元素

<Window x:Class="Wpf180706.Window10"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window10" Height="300" Width="300">
    <Window.Resources>
        <ControlTemplate x:Key="temp">
            <StackPanel Background="Yellow">
                <TextBox Name="txt1" Margin="6"></TextBox>
                <TextBox Name="txt2" Margin="6"></TextBox>
                <TextBox Name="txt3" Margin="6"></TextBox>
            </StackPanel>
            
        </ControlTemplate>
    </Window.Resources>
    
    <Grid>
        <StackPanel>
            <UserControl Name="uc" Template="{StaticResource temp}"></UserControl>
            <Button Click="btn_Click">btn</Button>
        </StackPanel>
    </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.Shapes;


namespace Wpf180706
{
    /// <summary>
    /// Interaction logic for Window10.xaml
    /// </summary>
    public partial class Window10 : Window
    {
        public Window10()
        {
            InitializeComponent();
        }


        private void btn_Click(object sender, RoutedEventArgs e)
        {
            TextBox txt = (TextBox)this.uc.Template.FindName("txt1",this.uc);
            txt.Text = "Hello";
            StackPanel sp = txt.Parent as StackPanel;
            ((TextBox)sp.Children[1]).Text = "Hello2";
            ((TextBox)sp.Children[2]).Text = "Hello3";
        }
    }
}