后进先出 stack用法

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            undostack = new Stack();
        }

        Stack undostack;
        private void button1_Click(object sender, EventArgs e)
        {
            undostack.Push(textBox1.Text);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (undostack.Count != 0)
            {
                string txt = (string)undostack.Pop();
                if (txt != null)
                {
                    textBox2.Text = txt;
                }
            }
        }
    }
}