switch-case

double a = double.Parse(textBox1.Text);
double b = double.Parse(textBox2.Text);


switch (comboBox1.SelectedIndex.ToString())
{
    case "0":
        double ret = Calc.Addition(a, b);
        break;

    case "1":
        double ret = Calc.Subtraction(a, b);
        break;

    case "2":
        double ret = Calc.Multiplication(a, b);
        break;

    case "3":
        double ret = Calc.Division(a, b);
        break;

    default:
        string ret = "Hibányos adatok.";
        break;
}

textBox3.Text = ret.ToString();