static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            bool existed;
            string guid = Marshal.GetTypeLibGuidForAssembly(Assembly.GetExecutingAssembly()).ToString();

            Mutex mutexObj = new Mutex(true, guid, out existed);
            if(existed)
                Application.Run(new MainForm());
            else
            {
                MessageBox.Show("Приложение уже запущенно");
                Application.Exit();
            }
        }
    }