How to make a program run only once. It could be any "hello world" program. It should say smth. (error, 2 console windows are not allowed) if the program is already running and shut down the second console window. (the first concole window should stay where it is)

Or this one:

static void Main()
        {            
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            string proc = Process.GetCurrentProcess().ProcessName;
            Process[] processes = Process.GetProcessesByName(proc);
            if (processes.Length > 1)
            {
                MessageBox.Show("Application already running...", "Warning", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            else
            {
                
            }
        }