试过网上介绍的几种方法,只有下面这个,能达到效果:
            bool b = false;
            System.Threading.Mutex m = new System.Threading.Mutex(true, Application.ProductName, out b);
            if (b)
            {
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new Form1());
                m.ReleaseMutex();
            }
            else
            {
                MessageBox.Show("短信评警后台程序已经启动!");
            }