Determine ports messages originated from

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C#

C#4.0

I have an windows service running, implementing an HTTP Listener with 4 prefixes, each with different port numbers. When a message is received from either of those ports, I would like to determine which port the message originated from. Any help would be appreciated.

Here is my code:

public void Listen(string[] prefix)
        {
            httpListener = new HttpListener();

            foreach (string s in prefix)
            {
                httpListener.Prefixes.Add(s);
            }

            //start the listener
            httpListener.Start();

            startListening();
        }

        public void startListening()
        {
            while (isTrue)
            {
                HttpListenerContext httpContext = httpListener.GetContext();
                HttpListenerRequest request = httpContext.Request;
                //string portNumber = ??????

                switch (portNumber)
                {
                    case 1:
                        //do something
                        break;
                    case 2:
                        //do something
                        break;
                    case 3:
                        //do something
                        break;
                    default:
                        Console.WriteLine("invalid port");
                        break;
                }
            }
        }