การใช้คำสั่ง หรือรูปแบบคำสั่ง Convert.ToInt32 ทำไมถึง error

private void buttonCalculate_Click(object sender, EventArgs e)
        {
            double price = Convert.ToDouble(textPrice.Text);
            int quantity = Convert.ToInt32(textQuantity.Text);

            double total = price * quantity;
            textTotal.Text = Convert.ToString(total);

        }

textTotal.Text = (string)total;

textTotal.Text = total.ToString();