WinForm

private static Mutex mutex = new Mutex(true, Application.ProductName);

[STAThread]
static void Main()
{
    if (mutex.WaitOne(TimeSpan.Zero, true))
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
        mutex.ReleaseMutex();
    }
}

WPF

private static Mutex mutex;

protected override void OnStartup(StartupEventArgs e)
{
    base.OnStartup(e);
    mutex = new Mutex(true, Assembly.GetExecutingAssembly().GetName().Name, out bool createdNew);
    if (!createdNew) Shutdown();
}