//在程序的main函数中加入以下代码

bool createdNew;

System.Threading.Mutex instance = new System.Threading.Mutex(true, "MutexName", out createdNew);

if (createdNew)

{

　　Application.Run(new LoginForm());

　　instance.ReleaseMutex();

}

else

{

　　Application.Exit();

}

//还可以写成以下形式，一个窗体只能启动一次

Form1 a = new Form1();

bool createdNew;

System.Threading.Mutex instance = new System.Threading.Mutex(true, "MutexName", out createdNew);

if (createdNew)

{

　　a.ShowDialog();

　　instance.ReleaseMutex();

}

else

{

　　a.Close();

}

private static bool HasRunning()

{

    Process currentProcess = Process.GetCurrentProcess();

    Process[] processCollection = Process.GetProcessesByName(currentProcess.ProcessName);

    foreach (Process p in processCollection)

    {

        if (p.Id == currentProcess.Id)  //检查ID是否相同

        {

            return true;

        }

    }

    return false;

}