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 != )

            {

                string txt = (string)undostack.Pop();

                if (txt != null)

                {

                    textBox2.Text = txt;

                }

            }

        }

    }

}