Question

I am writing a program calls “APPLICATION A” for the sake of simplicity which opens a socket connection to port 8881 on local host; later it spawns an exe application call “APPLICATION B” using the code blow:

        try
        {
            XmSam.Log("Spawning (" + process + ")");

            ProcessEx proc = new ProcessEx();

            proc.StartInfo.FileName = AppDomain.CurrentDomain.BaseDirectory + process + ".exe";
            proc.StartInfo.Arguments = parameters;
            proc.StartInfo.UseShellExecute = false;
            proc.Start();

            Program.Processes.Add(proc);

            XmSam.Log("Spawned " + proc.ProcessName);
        }
        catch (Exception ex)
        {
            XmSam.Log(ex);
        }

When I turn off APPLICATION A (either by stop the application or kill its process in window task manager), the APPLICATION B still runs which is fine. However, when I started APPLICATION A again I got the exception:

2012-10-18 16:21:23 - Only one usage of each socket address (protocol/network address/port) is normally permitted
2012-10-18 16:21:23 -    at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)

This error occured when APPLICATION A tried to open the socket to port 8881. This error would go away if I kill APPLICATION B. I don’t think it is because of APPLICATION B code because it has nothing such as the below code shows:

    [STAThread]
    static void Main()
    {
        while (true)
        {
        }
    }

I tried to look at MSDN under Process class but I did not see anything about this. I assumed when you spawned a process, that process is independent to the invocation application but apparently that is not the case. Do you know of any setting that let me spawn a process without having it holding onto the invocation application?

I know another way is to have code in APPLICATION A to kill APPLICATION B but that is not the idea solution especially in the case of APPLICATION A crashes unexpectedly.

Thank you.

Answer 1

I found the answer after more digging around. I find this excellent article

In it, the main explanation for this issue is as follow:

System.Diagnostics.Process.Start causes the child process to inherit handles from the parent process. That means, whatever process you spawn (e.g. Internet Explorer), that process gets all of the handles owned by your process! So even if you free up handles in your process, if the process you spawned is still running, your handles won’t really be freed because the child process is supposedly using them.

So with that in mind, all I have to do is to update my UseShellExecute flag to true.

proc.StartInfo.UseShellExecute = true;