Sometimes, it’s desirable to ensure that there is only ever one instance of your application running at any given time. Take Windows Live Messenger for instance – if you try to launch it whilst it is already running, it will just bring itself to the foreground instead.

Unfortunately, a lot of people try to recreate this behavior by simply checking if a process with the same name is currently running. As K. Scott Allen explains, this is not a good idea. The correct way to implement a single instance application, is to use a named mutex.

The word mutex is short for mutual exclusion, and is a synchronisation object that can only be owned by a single thread at any given time. Specifying a name for the mutex is optional – an unnamed mutex is scoped to the current process, while a named one is associated with an operating system object and can thus be used for interprocess synchronisation. Quite simply then, we can launch our application like this:

bool createdNew = true;
using (Mutex mutex = new Mutex(true, "MyApplicationName", out createdNew))
{
if (createdNew)
{
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new MainForm());
}
}

That ensures that there’s only a single instance of our application running. Now, the above code just ‘does nothing’ if the application is already running – it would be nice if it instead tried to give the main window focus. To do this, we need to find the process instance, and then pinvoke the SetForeGroundWindow method of the Win32 API. Our final Main method then looks like this:

static extern bool SetForegroundWindow(IntPtr hWnd);
/// <summary>
/// The main entry point for the application.
/// </summary>

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;
}
}
}
}
}

On a final note, I would urge you to read Raymond Chen’s excellent post «A single-instance program is it’s own denial of service», which talks about the security implications of implementing single-instance applications.