当我们要创建一个Tcp/UDP Server connection ,我们需要一个范围在1000到65535之间的端口 。但是本机一个端口只能一个程序监听，所以我们进行本地监听的时候需要检测端口是否被占用。命名空间System.Net.NetworkInformation下定义了一个名为IPGlobalProperties的类，我们使用这个类可以获取所有的监听连接，然后判断端口是否被占用.

//----------------------------------------------------------------------------- 
// Filename: FreePort.cs 
// 
// Description: Helper methods to find the next free UDP and TCP ports. 
// 
// History: 
// 28 Mar 2012    Aaron Clauson    Copied from http://www.mattbrindley.com/developing/windows/net/detecting-the-next-available-free-tcp-port/. 
//-----------------------------------------------------------------------------

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

namespace SIPSorcery.Sys.Net 
{ 
    public class FreePort 
    { 
        private const string PortReleaseGuid = "8875BD8E-4D5B-11DE-B2F4-691756D89593";

        /// <summary> 
        /// Check if startPort is available, incrementing and 
        /// checking again if it‘s in use until a free port is found 
        /// </summary> 
        /// <param name="startPort">The first port to check</param> 
        /// <returns>The first available port</returns> 
        public static int FindNextAvailableTCPPort(int startPort) 
        { 
            int port = startPort; 
            bool isAvailable = true;

            var mutex = new Mutex(false, 
                string.Concat("Global/", PortReleaseGuid)); 
            mutex.WaitOne(); 
            try 
            { 
                IPGlobalProperties ipGlobalProperties = 
                    IPGlobalProperties.GetIPGlobalProperties(); 
                IPEndPoint[] endPoints = 
                    ipGlobalProperties.GetActiveTcpListeners();

                do 
                { 
                    if (!isAvailable) 
                    { 
                        port++; 
                        isAvailable = true; 
                    }

                    foreach (IPEndPoint endPoint in endPoints) 
                    { 
                        if (endPoint.Port != port) continue; 
                        isAvailable = false; 
                        break; 
                    }

                } while (!isAvailable && port < IPEndPoint.MaxPort);

                if (!isAvailable) 
                    throw new ApplicationException("Not able to find a free TCP port.");

                return port; 
            } 
            finally 
            { 
                mutex.ReleaseMutex(); 
            } 
        }

        /// <summary> 
        /// Check if startPort is available, incrementing and 
        /// checking again if it‘s in use until a free port is found 
        /// </summary> 
        /// <param name="startPort">The first port to check</param> 
        /// <returns>The first available port</returns> 
        public static int FindNextAvailableUDPPort(int startPort) 
        { 
            int port = startPort; 
            bool isAvailable = true;

            var mutex = new Mutex(false, 
                string.Concat("Global/", PortReleaseGuid)); 
            mutex.WaitOne(); 
            try 
            { 
                IPGlobalProperties ipGlobalProperties = 
                    IPGlobalProperties.GetIPGlobalProperties(); 
                IPEndPoint[] endPoints = 
                    ipGlobalProperties.GetActiveUdpListeners();

                do 
                { 
                    if (!isAvailable) 
                    { 
                        port++; 
                        isAvailable = true; 
                    }

                    foreach (IPEndPoint endPoint in endPoints) 
                    { 
                        if (endPoint.Port != port) 
                            continue; 
                        isAvailable = false; 
                        break; 
                    }

                } while (!isAvailable && port < IPEndPoint.MaxPort);

                if (!isAvailable) 
                    throw new ApplicationException("Not able to find a free TCP port.");

                return port; 
            } 
            finally 
            { 
                mutex.ReleaseMutex(); 
            } 
        } 
    } 
} 

C#获取本机可用端口