Statement checker c# using forms and frontend

I’m just here for suggestions and code optimisations, what i could’ve done better, are there any better coding practices…
There’s github repo link if you’re interested in the code with a form too https://github.com/throtz/Operators
You basically have 2 textboxes in which you enter numbers and choose with a combobox what you want to do with those two numbers.

using System;
using System.Windows.Forms;

namespace Operators
{
    public partial class Operators : Form
    {
        public Operators()
        {
            InitializeComponent();
        }
        private void Form1_Load_1(object sender, EventArgs e)
        {
            comboBox.Items.AddRange(new string() { "+", "-", "*", "/", "%", ">", "<", "=", ">=", "<=" });
        }
        private void ButtonCheck_Click_1(object sender, EventArgs e)
        {
            if (sender is null)
            {
                throw new ArgumentNullException(nameof(sender));
            }

            int a = Convert.ToInt32(textBox1.Text);
            int b = Convert.ToInt32(textBox2.Text);

            Nah(a, b);
        }
        private void Nah(int a, int b)
        {
            switch (comboBox.SelectedItem.ToString())
            {
                case "+":
                    MessageBox.Show("Sum of thenumbers entered is: " + (a + b));
                    break;
                case "-":
                    MessageBox.Show("Subtraction of the numbers entered is: " + (a - b));
                    break;
                case "*":
                    MessageBox.Show("Multiplication of the numbers entered is: " + (a * b));
                    break;
                case "/":
                    MessageBox.Show("Division of the numbers entered is: " + (a / b));
                    break;
                case "%":
                    MessageBox.Show("Modulo the two numbers is: " + (a % b));
                    break;
                case ">":
                    MessageBox.Show("First number is bigger than the secound one " + (a > b));
                    break;
                case "<":
                    MessageBox.Show("First number is smaller than the secound one " + (a < b));
                    break;
                case "=":
                    MessageBox.Show("Numbers are equal: " + (a == b));
                    break;
                case ">=":
                    MessageBox.Show("First number is bigger or equal to number two " + (a >= b));
                    break;
                case "<=":
                    MessageBox.Show("First number is smaller or equal to number two " + (a <= b));
                    break;
                default:
                    MessageBox.Show("Illegal operation.");
                    break;
            }
        }
    }
}