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(); } } }
}