How can I create an HttpListener class on a random port in C#?

Question

I would like to create an application that serves web pages internally and can be run in multiple instances on the same machine. To do so, I would like to create an HttpListener that listens on a port that is: 1) Randomly selected 2) Currently unused

Essentially, what I would like is something like:

mListener = new HttpListener();
mListener.Prefixes.Add("http://*:0/");
mListener.Start();
selectedPort = mListener.Port;

How can I accomplish this?

Answer 1

How about something like this:

    static List<int> usedPorts = new List<int>();
    static Random r = new Random();

    public HttpListener CreateNewListener()
    {
        HttpListener mListener;
        int newPort = -1;
        while (true)
        {
            mListener = new HttpListener();
            newPort = r.Next(1025, 65535); // be nice, don't use ports bellow 1025
            if (usedPorts.Contains(newPort))
            {
                continue;
            }
            mListener.Prefixes.Add(string.Format("http://*:{0}/", newPort));
            try
            {
                mListener.Start();
            }
            catch
            {
                continue;
            }
            usedPorts.Add(newPort);
            break;
        }

        return mListener;
    }

I'm not sure how you would find all of the ports that are in use on that machine, but you should get an exception if you try to listen on a port that is already being used, in which case the method will simply pick another port.

Answer 2

TcpListener will find a random un-used port to listen on if you bind to port 0.

public static int GetRandomUnusedPort()
{
    var listener = new TcpListener(IPAddress.Any, 0);
    listener.Start();
    var port = ((IPEndPoint)listener.LocalEndpoint).Port;
    listener.Stop();
    return port;
}