[STAThread]
public static void Main()
{
    bool ret;
    System.Threading.Mutex mutex = new System.Threading.Mutex(true, Application.ProductName, out ret);
    if (ret)
    {
        System.Windows.Forms.Application.EnableVisualStyles();   //这两行实现   XP   可视风格   
        System.Windows.Forms.Application.DoEvents();
        System.Windows.Forms.Application.Run(new Main());
        //   Main   为你程序的主窗体，如果是控制台程序不用这句   
        mutex.ReleaseMutex();
    }
    else
    {
        MessageBox.Show(null, "有一个和本程序相同的应用程序已经在运行，请不要同时运行多个本程序。\n\n这个程序即将退出。", Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Warning);
        //   提示信息，可以删除。   
        Application.Exit();//退出程序   
    }
}

方法二：禁止多个进程运行,并当重复运行时激活以前的进程

[STAThread]
public static void Main()
{
    //Get   the   running   instance.   
    Process instance = RunningInstance();
    if (instance == null)
    {
        System.Windows.Forms.Application.EnableVisualStyles();   //这两行实现   XP   可视风格   
        System.Windows.Forms.Application.DoEvents();
        //There   isn‘t   another   instance,   show   our   form.   
        Application.Run(new Main());
    }
    else
    {
        //There   is   another   instance   of   this   process.   
        HandleRunningInstance(instance);
    }
}
public static Process RunningInstance()
{
    Process current = Process.GetCurrentProcess();
    Process[] processes = Process.GetProcessesByName(current.ProcessName);
    //Loop   through   the   running   processes   in   with   the   same   name   
    foreach (Process process in processes)
    {
        //Ignore   the   current   process   
        if (process.Id != current.Id)
        {
            //Make   sure   that   the   process   is   running   from   the   exe   file.   
            if (Assembly.GetExecutingAssembly().Location.Replace("/", "\\") == current.MainModule.FileName)
            {
                //Return   the   other   process   instance.   
                return process;
            }
        }
    }
    //No   other   instance   was   found,   return   null. 
    return null;
}

public static void HandleRunningInstance(Process instance)
{
    //Make   sure   the   window   is   not   minimized   or   maximized   
    ShowWindowAsync(instance.MainWindowHandle, WS_SHOWNORMAL);
    //Set   the   real   intance   to   foreground   window
    SetForegroundWindow(instance.MainWindowHandle);
}

[DllImport("User32.dll")]
private static extern bool ShowWindowAsync(IntPtr hWnd, int cmdShow);

[DllImport("User32.dll")]
private static extern bool SetForegroundWindow(IntPtr hWnd);

private const int WS_SHOWNORMAL = 1;


引用：https://www.cnblogs.com/lidj/archive/2012/07/06/2579897.html