Question

I have a single-instance app (c#, WPF, .net3.51). The check if the app is already instantiated is done via a Mutex. If the app is already running, I open a new window from within the already opened app-instance. This works fine so far.

However due to an app extension, I now must send the e.Args (or at least the first string of it) to the already running instance which resides in another process. How is this best done?

Additional Information
Currently I use a globaly registered Window-message that I send to all open apps via PostMessage (HWND_BROADCAST). My app looks for this message and opens a new window, if this message is received. An Idea would be to set a param of PostMessage. However I found a lot of bewildering information on this topic, therefore I had not the courage to go this way. Besides of that I thought of replacing the global PostMessage-call through another activation logic, since the global call seems to have some unlovely side-effects.

Answer 1

You could use named pipes, which were added to the BCL in .NET 3.5.

Create the named pipe server (argument receiver) in the already-running instance and create the named pipe client (argument sender) in the duplicate app. Then, send the arguments from the client to server.

Either end of the named pipe can be created in C/C++ if needed. See the Win32 CreateNamedPipe function.

Below is a simple example with the client and server both running in a single program (the “FD1AF2B4…” GUID below is just a unique identifier to avoid colliding with already-existing named pipes on the system).

class Program
{
    static void Main(string[] args)
    {
        Thread writerThread = new Thread(new ThreadStart(WriterThread));
        writerThread.Start();

        Thread readerThread = new Thread(new ThreadStart(ReaderThread));
        readerThread.Start();
    }

    static void ReaderThread()
    {
        NamedPipeServerStream server = new NamedPipeServerStream("FD1AF2B4-575A-46E0-8DF5-8AB368CF6645");
        server.WaitForConnection();

        using (var reader = new BinaryReader(server))
        {
            string arguments = reader.ReadString();
            Console.WriteLine("Received: {0}", arguments);
        }
    }

    static void WriterThread()
    {
        NamedPipeClientStream client = new NamedPipeClientStream("FD1AF2B4-575A-46E0-8DF5-8AB368CF6645");
        client.Connect(Timeout.Infinite);

        using (var writer = new BinaryWriter(client))
        {
            writer.Write("/foo /bar:33 /baz:quux");
        }
    }
}