program całka visual basic - wybór metody

Witam,
stworzyłem program w visual basicu do obliczania całek metoda trapezów, prostokątów i parabol. Jak dodać kod w programie abym w comboboxie mógł wyswietlac która metoda chce aby program liczył.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        
        double f (double x)
        {
            return Math.Pow(x, 2) - 2 * x + 3;
        }
        
        private void button1_Click(object sender, EventArgs e)
        {
            double a = Convert.ToDouble(textBoxA.Text);
            double b = Convert.ToDouble(textBoxB.Text);
            double N = Convert.ToDouble(textBoxN.Text);

            double xi,
                   Pi,
                     wartosc_calki = 0;

            double h = (b - a) / N;
            for (int i = 0; i < N; i++)
            {
                xi = a + h + i;
                Pi = h * f(xi);
//Pi = h * f(xi + h / 2); //met.prost. wz. otw
//Pi = h / 2 * (f(xi) + f(xi + h)); //met. trap. wz. zamk.
// Pi = h / 2 * (f(xi + h / 3) + f(xi + 2 * h / 3)); //met. trap wz otw
//Pi = h / 6 * (f(xi) + 4 * f(xi + h / 2) + f(xi + h));//met. parabol zamk.

                wartosc_calki += Pi;
            }
            textBoxWynik.Text = wartosc_calki.ToString();
        }
    }
}