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;
        }
    }
}

With the TcpListener approach I already got two Bluescreens:
Probably caused by : tcpip.sys ( tcpip!TcpDeliverLoopbackDataToClient+24c )

But... If you want to open empty port, you should let the system do the work for you:

var udp = new UdpClient(0, AddressFamily.InterNetwork);
int port = ((IPEndPoint)udp.Client.LocalEndPoint).Port;

Because... another process may open the port returning by GetAvailablePort() function, before you.

A shorter version:

public static int GetAvailablePort(int startingPort)
{
    if (startingPort > ushort.MaxValue) throw new ArgumentException($"Can't be greater than {ushort.MaxValue}", nameof(startingPort));
    var ipGlobalProperties = IPGlobalProperties.GetIPGlobalProperties();

    var connectionsEndpoints = ipGlobalProperties.GetActiveTcpConnections().Select(c => c.LocalEndPoint);
    var tcpListenersEndpoints = ipGlobalProperties.GetActiveTcpListeners();
    var udpListenersEndpoints = ipGlobalProperties.GetActiveUdpListeners();
    var portsInUse = connectionsEndpoints.Concat(tcpListenersEndpoints)
                                         .Concat(udpListenersEndpoints)
                                         .Select(e => e.Port);

    return Enumerable.Range(startingPort, ushort.MaxValue - startingPort + 1).Except(portsInUse).FirstOrDefault();
}

You could write the method like this

public static int GetAvailablePort(int startingPort)
{
    var properties = IPGlobalProperties.GetIPGlobalProperties();

    //getting active connections
    var tcpConnectionPorts = properties.GetActiveTcpConnections()
                        .Where(n => n.LocalEndPoint.Port >= startingPort)
                        .Select(n => n.LocalEndPoint.Port);

    //getting active tcp listners - WCF service listening in tcp
    var tcpListenerPorts = properties.GetActiveTcpListeners()
                        .Where(n => n.Port >= startingPort)
                        .Select(n => n.Port);

    //getting active udp listeners
    var udpListenerPorts = properties.GetActiveUdpListeners()
                        .Where(n => n.Port >= startingPort)
                        .Select(n => n.Port);

    var port = Enumerable.Range(startingPort, ushort.MaxValue)
        .Where(i => !tcpConnectionPorts.Contains(i))
        .Where(i => !tcpListenerPorts.Contains(i))
        .Where(i => !udpListenerPorts.Contains(i))
        .FirstOrDefault();

    return port;
}

not bad but didn't work

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;
    }
}

Just want to draw attention to @marbel82's comment. Finding an unused port number yourself has the problem that another process could steal the port before you bind to it. Suggest using your networking library's facility for letting the system pick an unused port and immediately bind it (e.g. specifying zero for the port number in the UdpClient constructor). Then read the port number it picked, if you still need to (UdpClient example).

I'm using to get a free port as @marbel82 suggested and it is working just fine