向客户端发送消息

server.PublishAsync(new MqttApplicationMessage()
   {
       Topic = "mytopic",
       QualityOfServiceLevel =             MQTTnet.Protocol.MqttQualityOfServiceLevel.ExactlyOnce,
       Retain = false,
       Payload = Encoding.UTF8.GetBytes("这是服务器消息！")
   }); ;