FreePortDetector.cs

namespace Example;

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Net.NetworkInformation;

public static class FreePortDetector
{
    private static readonly List<int> ReservedPorts = [];

    public static bool IsPortFree(int port)
    {
        if (ReservedPorts.Contains(port))
        {
            return false;
        }
        else
        {
            // http://stackoverflow.com/a/570126/107625

            var globalProperties = IPGlobalProperties.GetIPGlobalProperties();
            var informations = globalProperties.GetActiveTcpListeners();

            return informations.All(information => information.Port != port);
        }
    }

    public static void RemovePort(int port)
    {
        ReservedPorts.Remove(port);
    }

    public static int GetFreePort()
    {
        if (int.TryParse(ConfigurationManager.AppSettings[@"webserver.listenPort"], out var alt) && alt > 0)
        {
            alt += ReservedPorts.Count;
            if (IsPortFree(alt))
            {
                ReservedPorts.Add(alt);
                return alt;
            }
        }

        // --

        for (var i = 0; i < 10; ++i)
        {
            var port = Random.Shared.Next(9000, 15000);
            if (IsPortFree(port))
            {
                ReservedPorts.Add(port);
                return port;
            }
        }

        throw new("No free port could be found.");
    }
}