static class Program  
  {  
    private static Mutex m_Mutex;  

    [STAThread]  
    static void Main()  
    {  
        Application.EnableVisualStyles();  
        Application.SetCompatibleTextRenderingDefault(false);  
 
        bool isRunning;  
        m_Mutex = new Mutex(true, System.Diagnostics.Process.GetCurrentProcess().ProcessName, out isRunning);  
        if (isRunning)  
            Application.Run(new Form1());  
        else 
            MessageBox.Show("The application is already running.", Application.ProductName,  MessageBoxButtons.OK, MessageBoxIcon.Exclamation);  
    }  
}