Program.cs

using System;
using System.Threading;
using System.Windows.Forms;

namespace iRacingModeSwitcher
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            bool createdNew;
            using (Mutex mutex = new Mutex(true, "iRacingModeSwitcherMutex", out createdNew))
            {
                if (!createdNew)
                {
                    MessageBox.Show(
                        "iRacing Mode Switcher is already running.",
                        "Instance Detected",
                        MessageBoxButtons.OK,
                        MessageBoxIcon.Warning
                    );
                    return;
                }

                // Enables modern visual styles and sets up rendering
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);

                // Launches the main form (your GUI)
                Application.Run(new Form1());
            }
        }
    }
}