需求：在開發影象處理軟體過程，需要撤回上一步操作；

解決思路：每一步結果儲存到全域性list,在撤銷時讀取即可；

List<Bitmap> list_bmp = new List<Bitmap>();
        private void button1_Click(object sender, EventArgs e)
        {
             list_bmp.Add(threshImge.ToBitmap());
        }

        private void button2_Click(object sender, EventArgs e)
        {
             list_bmp.Add(threshImge.ToBitmap());
        }
        private void button8_Click(object sender, EventArgs e)
        {
            Bitmap bmp = list_bmp.ElementAt(list_bmp.Count-1);
            list_bmp.RemoveAt(list_bmp.Count - 1);
            pictureBox2.Image = bmp;
        }