public interface IHubConnectionContext<T>
 {
 T All { get; } // 代表所有客户端

 T AllExcept(params string[] excludeConnectionIds); // 除了参数中的所有客户端
 T Client(string connectionId); // 特定的客户端，这个方法也就是我们实现端对端聊天的关键
 T Clients(IList<string> connectionIds); // 参数中的客户端端
 T Group(string groupName, params string[] excludeConnectionIds); // 指定客户端组，这个也是实现群聊的关键所在
 T Groups(IList<string> groupNames, params string[] excludeConnectionIds);
 T User(string userId); // 特定的用户
 T Users(IList<string> userIds); // 参数中的用户
 }

public class ChatHub : Hub
 {
 // 静态属性
 public static List<UserInfo> OnlineUsers = new List<UserInfo>(); // 在线用户列表

 /// <summary>
 /// 登录连线
 /// </summary>
 /// <param name="userId">用户Id</param>
 /// <param name="userName">用户名</param>
 public void Connect(string userId, string userName)
 {
  var connnectId = Context.ConnectionId;

  if (OnlineUsers.Count(x => x.ConnectionId == connnectId) == 0)
  {
  if (OnlineUsers.Any(x => x.UserId == userId))
  {
   var items = OnlineUsers.Where(x => x.UserId == userId).ToList();
   foreach (var item in items)
   {
   Clients.AllExcept(connnectId).onUserDisconnected(item.ConnectionId, item.UserName);
   }
   OnlineUsers.RemoveAll(x => x.UserId == userId);
  }

  //添加在线人员
  OnlineUsers.Add(new UserInfo
  {
   ConnectionId = connnectId,
   UserId = userId,
   UserName = userName,
   LastLoginTime = DateTime.Now
  });
  }

  // 所有客户端同步在线用户
  Clients.All.onConnected(connnectId, userName, OnlineUsers);
 }


 /// <summary>
 /// 发送私聊
 /// </summary>
 /// <param name="toUserId">接收方用户连接ID</param>
 /// <param name="message">内容</param>
 public void SendPrivateMessage(string toUserId, string message)
 {
  var fromUserId = Context.ConnectionId;

  var toUser = OnlineUsers.FirstOrDefault(x => x.ConnectionId == toUserId);
  var fromUser = OnlineUsers.FirstOrDefault(x => x.ConnectionId == fromUserId);

  if (toUser != null && fromUser != null)
  { 
  // send to 
  Clients.Client(toUserId).receivePrivateMessage(fromUserId, fromUser.UserName, message);

  // send to caller user
  // Clients.Caller.sendPrivateMessage(toUserId, fromUser.UserName, message);
  }
  else
  {
  //表示对方不在线
  Clients.Caller.absentSubscriber();
  }
 }

 /// <summary>
 /// 断线时调用
 /// </summary>
 /// <param name="stopCalled"></param>
 /// <returns></returns>
 public override Task OnDisconnected(bool stopCalled)
 {
  var user = OnlineUsers.FirstOrDefault(u => u.ConnectionId == Context.ConnectionId);

  // 判断用户是否存在,存在则删除
  if (user == null) return base.OnDisconnected(stopCalled);

  Clients.All.onUserDisconnected(user.ConnectionId, user.UserName); //调用客户端用户离线通知
  // 删除用户
  OnlineUsers.Remove(user);


  return base.OnDisconnected(stopCalled);
 }
 }

 <script type="text/javascript">
 var systemHub = $.connection.chatHub;
 / 连接IM服务器成功
 // 主要是更新在线用户
 systemHub.client.onConnected = function (id, userName, allUsers) {
  var node = chatCore.node, myf = node.list.eq(0), str = '', i = 0;
  myf.addClass('loading');
  onlinenum = allUsers.length;
  if (onlinenum > 0) {
  str += '<li class="ChatCore_parentnode ChatCore_liston">'
   + '<h5><i></i><span class="ChatCore_parentname">在线用户</span><em class="ChatCore_nums">（' + onlinenum + '）</em></h5>'
   + '<ul id="ChatCore_friend_list" class="ChatCore_chatlist">';
  for (; i < onlinenum; i++) {
   str += '<li id="userid-' + allUsers[i].UserID + '" data-id="' + allUsers[i].ConnectionId + '" class="ChatCore_childnode" type="one"><img src="/Content/Images/001.jpg?' + allUsers[i].UserID + '" class="ChatCore_oneface"><span class="ChatCore_onename">' + allUsers[i].UserName + '(' + ')</span><em class="ChatCore_time">' + allUsers[i].LoginTime + '</em></li>';
  }
  str += '</ul></li>';
  myf.html(str);
  } else {
  myf.html('<li class="ChatCore_errormsg">没有任何数据</li>');
  }
  myf.removeClass('loading');
 };
 //消息传输
 chatCore.transmit = function () {
 var node = chatCore.node, log = {};
 node.sendbtn = $('#ChatCore_sendbtn');
 node.imwrite = $('#ChatCore_write');

 //发送
 log.send = function () {
  var data = {
  content: node.imwrite.val(),
  id: chatCore.nowchat.id,
  sign_key: '', //密匙
  _: +new Date
  };

  if (data.content.replace(/\s/g, '') === '') {
  layer.tips('说点啥呗！', '#ChatCore_write', 2);
  node.imwrite.focus();
  } else {
  //此处皆为模拟
  var keys = chatCore.nowchat.type + chatCore.nowchat.id;

  //聊天模版
  log.html = function (param, type) {
   return '<li class="' + (type === 'me' ? 'ChatCore_chateme' : '') + '">'
   + '<div class="ChatCore_chatuser">'
    + function () {
    if (type === 'me') {
     return '<span class="ChatCore_chattime">' + param.time + '</span>'
      + '<span class="ChatCore_chatname">' + param.name + '</span>'
      + '<img src="' + param.face + '" >';
    } else {
     return '<img src="' + param.face + '" >'
      + '<span class="ChatCore_chatname">' + param.name + '</span>'
      + '<span class="ChatCore_chattime">' + param.time + '</span>';
    }
    }()
   + '</div>'
   + '<div class="ChatCore_chatsay">' + param.content + '<em class="ChatCore_zero"></em></div>'
   + '</li>';
  };

  log.imarea = chatCore.chatbox.find('#ChatCore_area' + keys);

  log.imarea.append(log.html({
   time: new Date().toLocaleString(),
   name: config.user.name,
   face: config.user.face,
   content: data.content
  }, 'me'));
  node.imwrite.val('').focus();
  log.imarea.scrollTop(log.imarea[0].scrollHeight);

  // 调用服务端sendPrivateMessage方法来转发消息
  systemHub.server.sendPrivateMessage(chatCore.nowchat.id, data.content);
  }

 };
 node.sendbtn.on('click', log.send);

 node.imwrite.keyup(function (e) {
  if (e.keyCode === 13) {
  log.send();
  }
 });
 };

 //用户离线
 systemHub.client.onUserDisconnected = function (id, userName) {
  onlinenum = onlinenum - 1;
  $(".ChatCore_nums").html("（" + onlinenum + "）");
  $("#ChatCore_friend_list li[data-id=" + id + "]").remove();
 };
 // 启动连接
 $.connection.hub.start().done(function () {
  systemHub.server.connect(userid, username); // 调用服务端connect方法
 });
 </script>