server的代码public abstract class Server {static readonly ILog logger = LogManager.GetLogger(typeof(Server));public int Port { get; set; }public event ClientEventHandler OnClientAcceptEvent;public event ClientEventHandler OnClientOnlineEvent;public event ClientEventHandler OnClientRemoveEvent;private bool bStarted;static private int NextClientId = 0;private TcpListener _listener;protected Dictionary<int, Client> id2client = new Dictionary<int, Client>();private List<Client> noHeartBeatClients = new List<Client>();public bool HasHeartBeat;private int CheckHeartBeatInterval = 30 * 1000;// msprotected System.Timers.Timer CheckHeartBeatTimer = new System.Timers.Timer();private object id2clientCS = new object();private object noHeartBeatClientsCS = new object();private Server() {CheckHeartBeatTimer.Elapsed += (o1, a1) => {if(HasHeartBeat == false) return;List<Client> kickClientList = new List<Client>();lock(id2clientCS) {try {DateTime now = DateTime.Now;foreach(KeyValuePair<int, Client> pair in id2client) {try {Client client = pair.Value;TimeSpan offset = now - client.LastHeartBeat;if(client != null && client.State == Client.ConnectionState.Connected && offset.TotalMilliseconds > CheckHeartBeatInterval) {kickClientList.Add(pair.Value);logger.InfoFormat("检测到心跳超时: [IP]{0}, [ID]{1}, [Time]{2}", client.ClientIpAddress, pair.Key, DateTime.Now.ToString());}} catch { }}} catch(Exception ex) {logger.WarnFormat("心跳检测时发生异常: \n{0}", ex);}}kickClientList.ForEach(p => p.Close());lock(noHeartBeatClientsCS) {kickClientList.ForEach(c => noHeartBeatClients.RemoveAll(p => p.Id == c.Id));}};}public Server(int port): this() {this.Port = port;}public List<Client> Clients {get {List<Client> result = new List<Client>();lock(id2clientCS) {foreach(Client each in id2client.Values) {result.Add(each);}}return result;}}public virtual void Open() {_listener = new TcpListener(Port);logger.InfoFormat("Server#Open port={0}", Port);try {_listener.Start();if(HasHeartBeat) {CheckHeartBeatTimer.Stop();CheckHeartBeatTimer.Interval = CheckHeartBeatInterval;CheckHeartBeatTimer.Start();}_listener.BeginAcceptTcpClient(new AsyncCallback(OnAccept), null);bStarted = true;} catch(SocketException ex) {logger.WarnFormat("服务器监听发生异常:{0}\nSocket ErrorCode: {1}\n提示:请检查端口是否已被占用", ex.Message, ex.ErrorCode);throw ex;} catch(Exception ex) {logger.Warn(ex);throw ex;}}public virtual void Close() {try {if(HasHeartBeat) {CheckHeartBeatTimer.Stop();}_listener.Stop();bStarted = false;lock(id2clientCS) {foreach(Client each in id2client.Values) {try { if(each != null)each.Close(); } catch { }}id2client.Clear();}} catch(Exception ex) {logger.Warn(ex);throw ex;}}private void OnAccept(IAsyncResult ar) {try {Client client = CreateClient(NextClientId++, _listener.EndAcceptTcpClient(ar), this);client.LastHeartBeat = DateTime.Now;client.PostOfflineEvent += (obj, args) => RemoveClient(client);lock(id2clientCS) {id2client.Add(client.Id, client);}lock(noHeartBeatClientsCS) {noHeartBeatClients.Add(client);}if(OnClientAcceptEvent != null) OnClientAcceptEvent(client);} catch(ObjectDisposedException) { } catch(Exception ex) {logger.Warn(ex);} finally {try {_listener.BeginAcceptTcpClient(new AsyncCallback(OnAccept), null);} catch(Exception) {// ignore}}}protected abstract Client CreateClient(int id, TcpClient tcpClient, Server server);public void RemoveClient(Client client) {if(bStarted == false) return;lock(id2clientCS) {id2client.Remove(client.Id);}if(OnClientRemoveEvent != null) OnClientRemoveEvent(client);}public void GetHeartBeat(Client client) {if(HasHeartBeat == false) return;client.LastHeartBeat = DateTime.Now;lock(noHeartBeatClientsCS) {int index = noHeartBeatClients.FindIndex(p => p.Id == client.Id);if(index == -1) return;try {if(OnClientOnlineEvent != null) OnClientOnlineEvent(client);} catch(Exception ex) {logger.Warn(ex);}noHeartBeatClients.RemoveAt(index);}}
}

-----解决方案--------------------------------------------------------
在定时器里隔段时间就发送几个字节的数据。如果3次没有返回则断开 
------解决方案--------------------------------------------------------
客户端:
30秒发送一个心跳包到服务器

服务器:
接收到心跳包,更新LastHeartbeatTime
并且有一个线程,一分钟扫描一次,如果LastHeartbeatTime超过一分钟没更新的视为下线

------解决方案--------------------------------------------------------

模拟心跳的机制
使用直接调用函数模拟心跳,没有涉及到socket
写得不好,不要太挑剔

using System;
using System.Collections.Generic;
using System.Threading;namespace ConsoleApplication1
{// 客户端离线委托public delegate void ClientOfflineHandler(ClientInfo client);// 客户端上线委托public delegate void ClientOnlineHandler(ClientInfo client);public class Program{/// <summary>/// 客户端离线提示/// </summary>/// <param name="clientInfo"></param>private static void ClientOffline(ClientInfo clientInfo){Console.WriteLine(String.Format("客户端{0}离线，离线时间：\t{1}", clientInfo.ClientID, clientInfo.LastHeartbeatTime));}/// <summary>/// 客户端上线提示/// </summary>/// <param name="clientInfo"></param>private static void ClientOnline(ClientInfo clientInfo){Console.WriteLine(String.Format("客户端{0}上线，上线时间：\t{1}", clientInfo.ClientID, clientInfo.LastHeartbeatTime));}static void Main(){// 服务端Server server = new Server();// 服务端离线事件server.OnClientOffline += ClientOffline;// 服务器上线事件server.OnClientOnline += ClientOnline;// 开启服务器server.Start();// 模拟100个客户端Dictionary<Int32, Client> dicClient = new Dictionary<Int32, Client>();for (Int32 i = 0; i < 100; i++){// 这里传入server只是为了方便而已Client client = new Client(i + 1, server);dicClient.Add(i + 1, client);// 开启客户端client.Start();}System.Threading.Thread.Sleep(1000);while (true){Console.WriteLine("请输入要离线的ClientID,输入0则退出程序:");String clientID = Console.ReadLine();if (!String.IsNullOrEmpty(clientID)){Int32 iClientID = 0;Int32.TryParse(clientID, out iClientID);if (iClientID > 0){Client client;if (dicClient.TryGetValue(iClientID, out client)){// 客户端离线client.Offline = true;}}else{return;}}}}}/// <summary>/// 服务端/// </summary>public class Server{public event ClientOfflineHandler OnClientOffline;public event ClientOnlineHandler OnClientOnline;private Dictionary<Int32, ClientInfo> _DicClient;/// <summary>/// 构造函数/// </summary>public Server(){_DicClient = new Dictionary<Int32, ClientInfo>(100);            }/// <summary>/// 开启服务端/// </summary>public void Start(){// 开启扫描离线线程Thread t = new Thread(new ThreadStart(ScanOffline));t.IsBackground = true;t.Start();}/// <summary>/// 扫描离线/// </summary>private void ScanOffline(){while (true){// 一秒扫描一次System.Threading.Thread.Sleep(1000);lock (_DicClient){foreach (Int32 clientID in _DicClient.Keys){ClientInfo clientInfo = _DicClient[clientID];// 如果已经离线则不用管if (!clientInfo.State){continue;}// 判断最后心跳时间是否大于3秒TimeSpan sp = System.DateTime.Now - clientInfo.LastHeartbeatTime;if (sp.Seconds >= 3){// 离线，触发离线事件if (OnClientOffline != null){OnClientOffline(clientInfo);}// 修改状态clientInfo.State = false;}}}}}/// <summary>/// 接收心跳包/// </summary>/// <param name="clientID">客户端ID</param>public void ReceiveHeartbeat(Int32 clientID){lock (_DicClient){ClientInfo clientInfo;if (_DicClient.TryGetValue(clientID, out clientInfo)){// 如果客户端已经上线，则更新最后心跳时间clientInfo.LastHeartbeatTime = System.DateTime.Now;}else{// 客户端不存在，则认为是新上线的clientInfo = new ClientInfo();clientInfo.ClientID = clientID;clientInfo.LastHeartbeatTime = System.DateTime.Now;clientInfo.State = true;_DicClient.Add(clientID, clientInfo);// 触发上线事件if (OnClientOnline != null){OnClientOnline(clientInfo);}}}}}/// <summary>/// 客户端/// </summary>public class Client{public Server Server;public Int32 ClientID;public Boolean Offline;/// <summary>/// 构造函数/// </summary>/// <param name="clientID"></param>/// <param name="server"></param>public Client(Int32 clientID, Server server){ClientID = clientID;Server = server;Offline = false;}/// <summary>/// 开启客户端/// </summary>public void Start(){// 开启心跳线程Thread t = new Thread(new ThreadStart(Heartbeat));t.IsBackground = true;t.Start();}/// <summary>/// 向服务器发送心跳包/// </summary>private void Heartbeat(){while (!Offline){// 向服务端发送心跳包Server.ReceiveHeartbeat(ClientID);System.Threading.Thread.Sleep(1000);}}}/// <summary>/// 客户端信息/// </summary>public class ClientInfo{// 客户端IDpublic Int32 ClientID;// 最后心跳时间public DateTime LastHeartbeatTime;// 状态public Boolean State;}
}