Request obs = Request.NewGet(); obs.URI = new Uri("coap://127.0.0.1/time"); obs.MarkObserve(); Observe fail. ???

[cwf689]

I use CoAP.Net 1.04.
server code：
//----------------------------------------------
//Resource code:
class TimeResource : Resource
{
private System.Threading.Timer _timer;
private DateTime _now;

  public TimeResource(String name)
      : base(name)
  {
      Attributes.Title = "GET the current time";
      Attributes.AddResourceType("CurrentTime");
      Observable = true;

      _timer = new System.Threading.Timer(Timed, null, 0, 10000);
  }

  private void Timed(Object o)
  {
      _now = DateTime.Now;
      Changed();
      Debug.WriteLine($"{_now},数值改变");
  }

  protected override void DoGet(CoapExchange exchange)
  {
      exchange.Respond(StatusCode.Content, _now.ToString(), MediaType.TextPlain);
      Debug.WriteLine($"{DateTime.Now},数值改变时间：{_now.ToString()}");
  }              CoapServer server = new CoapServer();

//----------------------------------------------
//start server code:
server.Add(new TimeResource("time"));
server.Start();
//----------------------------------------------
client code:
public void Observe()
{
Request obs = Request.NewGet();
obs.URI = new Uri("coap://127.0.0.1/time");
obs.MarkObserve();
// add event handler
obs.Respond += delegate (Object? sender, ResponseEventArgs e)
{
// this is a new notification
Response response = e.Response;
// TODO update window state here
// you may need to switch to the UI thread first
};
//obs.SetPayload("data from client");
obs.Send();
}
//----------------------------------------------
question:
obs.Respond += delegate (Object? sender, ResponseEventArgs e) ;
delegate will no any response
but i remove "obs.MarkObserve();" ,delegate will response received
so i can not Observe. what's wrong?
help me! thanks