public static double Round(double d, int i)
        {
            if (d >= 0)
            {
                d += 5 * Math.Pow(10, -(i + 1));//求指定次數的指定次冪
            }
            else
            {
                d += 5 * Math.Pow(10, -(i + 1));
            }
            string str = d.ToString();
            string[] strs = str.Split('.');
            int idot = str.IndexOf('.');
            string prestr = strs[0];
            string poststr = strs[1];
            if (poststr.Length > i)
            {
                poststr = str.Substring(idot + 1, i);//截取需要位數
            }
            if (poststr.Length <= 2)
            {
                poststr = poststr + "0";
            }
            string strd = prestr + "." + poststr;
            d = double.Parse(strd);//將字符串轉換為雙精度實數
            return d;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBox3.Text=Convert.ToString(Math.Round(Convert.ToDouble(textBox1.Text.Trim()),Convert.ToInt16(textBox2.Text.Trim())));
        }