<Window x:Class="WpfApplication1.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>
        <TextBox Name="box1" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" FontSize="24" Foreground="Blue" Height="50" Margin="20,134,345,136" RenderTransformOrigin="0.26,0.18">aaa</TextBox>
        <Button x:Name="btn" Click="btn_click" Content="Button" HorizontalAlignment="Left" Margin="201,149,0,0" VerticalAlignment="Top" Width="75"/>
        <TextBox Name="box2" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" FontSize="24" Foreground="Blue" Height="50" Margin="340,134,25,136" RenderTransformOrigin="0.26,0.18"></TextBox>
    </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 WpfApplication1
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        public void btn_click(object sender, RoutedEventArgs e)
        {
            box2.Text = "bbb";
        }
    }
}