Find an Available Port with C#

AvailablePorts.cs

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

namespace AvailablePort
{
    class Program
    {
        static void Main(string[] args)
        {

            Console.WriteLine(GetAvailablePort(1000).ToString());
            Console.ReadLine();
        }

        /// <summary>
        /// checks for used ports and retrieves the first free port
        /// </summary>
        /// <returns>the free port or 0 if it did not find a free port</returns>
        public static int GetAvailablePort(int startingPort)
        {
            IPEndPoint[] endPoints;
            List<int> portArray = new List<int>();

            IPGlobalProperties properties = IPGlobalProperties.GetIPGlobalProperties();

            //getting active connections
            TcpConnectionInformation[] connections = properties.GetActiveTcpConnections();
            portArray.AddRange(from n in connections
                               where n.LocalEndPoint.Port >= startingPort
                               select n.LocalEndPoint.Port);

            //getting active tcp listners - WCF service listening in tcp
            endPoints = properties.GetActiveTcpListeners();
            portArray.AddRange(from n in endPoints
                               where n.Port >= startingPort
                               select n.Port);

            //getting active udp listeners
            endPoints = properties.GetActiveUdpListeners();
            portArray.AddRange(from n in endPoints
                               where n.Port >= startingPort
                               select n.Port);

            portArray.Sort();

            for (int i = startingPort; i < UInt16.MaxValue; i++)
                if (!portArray.Contains(i))
                    return i;

            return 0;
        }
    }
}

int port = startPort;

        // Try ports until available port is found
        while (IPGlobalProperties
            .GetIPGlobalProperties()
            .GetActiveTcpListeners()
            .Any(x => x.Port == port))
        {
            if (port > port + portRange)
            {
                throw new SystemException($"Couldn't find open port within range {port - portRange} - {port}.");
            }

            port++;
        }

public static class IpUtilities
{
    private const ushort MIN_PORT = 1;
    private const ushort MAX_PORT = UInt16.MaxValue;
    public static int? GetAvailablePort(ushort lowerPort = MIN_PORT, ushort upperPort = MAX_PORT)
    {
        var ipProperties = IPGlobalProperties.GetIPGlobalProperties();
        var usedPorts = Enumerable.Empty<int>()
            .Concat(ipProperties.GetActiveTcpConnections().Select(c => c.LocalEndPoint.Port))
            .Concat(ipProperties.GetActiveTcpListeners().Select(l => l.Port))
            .Concat(ipProperties.GetActiveUdpListeners().Select(l => l.Port))
            .ToHashSet();
        for (int port = lowerPort; port <= upperPort; port++)
        {
            if (!usedPorts.Contains(port)) return port;
        }
        return null;
    }
}