Question

I have a situation where I need to have only one instance of a program running at the same time.

This would be trivial like this:

class OneAtATimePlease
{
  static void Main()
  {
    // Naming a Mutex makes it available computer-wide. Use a name that's
    // unique to your company and application (e.g., include your URL).

    using (var mutex = new Mutex (false, "oreilly.com OneAtATimeDemo"))
    {
      // Wait a few seconds if contended, in case another instance
      // of the program is still in the process of shutting down.

      if (!mutex.WaitOne (TimeSpan.FromSeconds (3), false))
      {
        Console.WriteLine ("Another app instance is running. Bye!");
        return;
      }
      RunProgram();
    }
  }

  static void RunProgram()
  {
    Console.WriteLine ("Running. Press Enter to exit");
    Console.ReadLine();
  }
}

Except for the small detail, that I need the EXISTING process to terminate, not the new one.

I tried making a semaphore the existing process could listen to after grabbing the above mutex, but because I want it to wait for the semaphore then I can end up in a situation where the semaphore is always signaled and thus it doesn’t work.

Anyone have a good idea on how to solve this problem?

Answer 1

You need inter-process communication, to send a signal to the existing application. For C#, see IPC Mechanisms in C# – Usage and Best Practices.