"；OnDisconnected():找不到合适的方法来覆盖“-”信号R

我一直在尝试通过遵循CodeProject上的"Asp.Net SignalR聊天室"教程来实现聊天室(http://www.codeproject.com/Articles/562023/Asp-Net-SignalR-Chat-Room)。然而，我得到了错误"Hubs.ChatHubs.OnDisconnected（）：找不到合适的方法来覆盖"

ChatHub类别：

public class ChatHub : Hub
{
    #region Data Members
    static List<UserDetail> ConnectedUsers = new List<UserDetail>();
    static List<MessageDetail> CurrentMessage = new List<MessageDetail>();
    #endregion
    #region Methods
    public void Connect(string userName)
    {
        var id = Context.ConnectionId;

        if (ConnectedUsers.Count(x => x.ConnectionId == id) == 0)
        {
            ConnectedUsers.Add(new UserDetail { ConnectionId = id, UserName = userName });
            // send to caller
            Clients.Caller.onConnected(id, userName, ConnectedUsers, CurrentMessage);
            // send to all except caller client
            Clients.AllExcept(id).onNewUserConnected(id, userName);
        }
    }
    public void SendMessageToAll(string userName, string message)
    {
        // store last 100 messages in cache
        AddMessageinCache(userName, message);
        // Broad cast message
        Clients.All.messageReceived(userName, message);
    }
    public void SendPrivateMessage(string toUserId, string message)
    {
        string fromUserId = Context.ConnectionId;
        var toUser = ConnectedUsers.FirstOrDefault(x => x.ConnectionId == toUserId);
        var fromUser = ConnectedUsers.FirstOrDefault(x => x.ConnectionId == fromUserId);
        if (toUser != null && fromUser != null)
        {
            // send to 
            Clients.Client(toUserId).sendPrivateMessage(fromUserId, fromUser.UserName, message);
            // send to caller user
            Clients.Caller.sendPrivateMessage(toUserId, fromUser.UserName, message);
        }
    }
    public override System.Threading.Tasks.Task OnDisconnected()
    {
        var item = ConnectedUsers.FirstOrDefault(x => x.ConnectionId == Context.ConnectionId);
        if (item != null)
        {
            ConnectedUsers.Remove(item);
            var id = Context.ConnectionId;
            Clients.All.onUserDisconnected(id, item.UserName);
        }
        return base.OnDisconnected();
    }

    #endregion
    #region private Messages
    private void AddMessageinCache(string userName, string message)
    {
        CurrentMessage.Add(new MessageDetail { UserName = userName, Message = message });
        if (CurrentMessage.Count > 100)
            CurrentMessage.RemoveAt(0);
    }
    #endregion
}

关于为什么会发生这种情况，有什么线索吗？