private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                account at  = new account();
                int     M   = int.Parse(txtNo1.Text.Trim());
                int     N   = int.Parse(txtNo2.Text.Trim());
                string  str = comboBox1.Text;
                switch (str)
                {
                case "加": txtResult.Text = at.addition(M, N).ToString(); break;

                case "减": txtResult.Text = at.subtration(M, N).ToString(); break;

                case "乘": txtResult.Text = at.multiplication(M, N).ToString(); break;

                case "除": txtResult.Text = at.division(M, N).ToString(); break;
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }