C#点击按钮，文本框数值随之增加

C# 文本框显示数值000000
点击按钮数值变为000001
再次点击按钮数值变为000002
点击至10次时，数值变为000010

   int num = 0;
        private void button1_Click(object sender, EventArgs e)
        {
            num++;
            this.textBox1.Text = num.ToString().PadLeft(6,'0');
        }