程序运行在交换时不能显示，寻求语句问题

using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 单分支结构
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
      

         private void textBox1_TextChanged(object sender, EventArgs e)
        {
            String x =textBox1.Text;
           
        }
      

        private void textBox2_TextChanged(object sender, EventArgs e)
        {
            String y = textBox2.Text;
           
        }

        private void button1_Click(object sender, EventArgs e)
        {
            String z;
            String x = textBox1.Text;
            String y = textBox2.Text;
            z = x;
            x = y;
            y = z;
            this.listBox1.Text =x;
        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

      

      
    }
}