Google is your friend. I googled "c# single app instance" and found this code. I don't know if it works, but it probably does:
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool SetForegroundWindow(IntPtr hWnd);
[STAThread]
static void Main()
{
bool createdNew = true;
using (Mutex mutex = new Mutex(true, "MyApplicationName", out createdNew))
{
if (createdNew)
{
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new MainForm());
}
else
{
Process current = Process.GetCurrentProcess();
foreach (Process process in Process.GetProcessesByName(current.ProcessName))
{
if (process.Id != current.Id)
{
SetForegroundWindow(process.MainWindowHandle);
break;
}
}
}
}
}
BTW, stop using "Plzzzzzzzzzz" in your messages. It just pisses everyone here off.