“未连接”异步套接字可防止应用程序关闭

Question

我使用BeginConnect（）编写了一个小的c＃类来检查TCP连接（连接到TCP套接字成功）。它似乎运行良好，如果有问题的端点可用且TCP会话实际连接。

然而，当端点没有监听并且连接超时时，会发生一些奇怪的事情：该类保持“挂起”大约15到20秒。这与默认超时值一致。

但是，当超时触发时我调用socket.Close（） - MSDN表示取消所有异步操作。

为什么当连接失败时，类会阻止应用程序关闭？我做错了什么？

    public class CheckTCP
{
    #region "Member Variables"

        // socket connect timeout value
        private int _timeout = 2000;

        // check complete event
        public event EventHandler<CheckCompletedEventArgs> CheckCompleted;

        // perform TCP connect check
        public void PerformCheck(EndPoint Socket)
        {
            StateObject state = new StateObject();
            state.Me = this;
            state.WorkSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            state.TimeoutTimer = new Timer();
            state.TimeoutTimer.Interval = _timeout;
            state.TimeoutTimer.Elapsed += (sender, e) => timeout_elapsed(sender, e, state);
            state.TimeoutTimer.Start();
            state.WorkSocket.SendTimeout = _timeout;
            state.WorkSocket.ReceiveTimeout = _timeout;
            state.WorkSocket.BeginConnect(Socket, new AsyncCallback(ConnectCallback), state);
        }

        // connection callback
        private static void ConnectCallback(IAsyncResult ar)
        {
            StateObject state = (StateObject) ar.AsyncState;
            state.TimeoutTimer.Stop();
            state.TimeoutTimer.Dispose();

            CheckCompletedEventArgs ea = new CheckCompletedEventArgs();

            if (state.WorkSocket.Connected)
            {
                state.WorkSocket.EndConnect(ar);
                state.WorkSocket.Close();
                ea.Success = true;
                state.Me.OnCheckCompleted(ea);
            }
            else
            {
                ea.Success = false;
                state.Me.OnCheckCompleted(ea);
            }

            state.WorkSocket.Dispose();
            state.Me = null;
        }

        // timeout callback
        private void timeout_elapsed(object sender, EventArgs e, StateObject state)
        {
            state.TimeoutTimer.Stop();
            state.WorkSocket.Close();
        }
    #endregion

        // raiseevent helper
        protected virtual void OnCheckCompleted(CheckCompletedEventArgs e)
        {
            // raise the event
            EventHandler<CheckCompletedEventArgs> handler = CheckCompleted;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        // checkcompleted event args class
        public class CheckCompletedEventArgs : EventArgs
        {
            public bool Success { set; get; }
        }

        // async state object
        public class StateObject
        {
            public EndPoint EndpointSocket { set; get; }
            public Socket WorkSocket { set; get; }
            public Timer TimeoutTimer { set; get; }
            public CheckTCP Me { set; get; }
        }

}