订阅消息

• 引用的MQTTnet最新4.1.0版本，新增MqttClientService类，实例化client。然后连接到服务端后订阅topic

using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Protocol;
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;
namespace MQTTClient
{
    public class MqttClientService
    {
        public static IMqttClient _mqttClient;
        public IMqttClient MqttClientStart(string ip, int port, string username, string pwd, string clientId)
        {
            var optionsBuilder = new MqttClientOptionsBuilder()
                .WithTcpServer(ip, port) // 要访问的mqtt服务端的 ip 和 端口号
                .WithCredentials(username, pwd) // 要访问的mqtt服务端的用户名和密码
                .WithClientId(clientId) // 设置客户端id
                .WithCleanSession()
                .WithTls(new MqttClientOptionsBuilderTlsParameters
                {
                    UseTls = false  // 是否使用 tls加密
                });
            var clientOptions = optionsBuilder.Build();
            _mqttClient = new MqttFactory().CreateMqttClient();
            //_mqttClient.ConnectedAsync += _mqttClient_ConnectedAsync; // 客户端连接成功事件
            //_mqttClient.DisconnectedAsync += _mqttClient_DisconnectedAsync; // 客户端连接关闭事件
            //_mqttClient.ApplicationMessageReceivedAsync += _mqttClient_ApplicationMessageReceivedAsync; // 收到消息事件
            _mqttClient.ConnectAsync(clientOptions);
            return _mqttClient;
        }
    }
}

using MQTTnet.Client;
using MQTTnet.Protocol;
using System;
using System.Text;
using System.Threading.Tasks;
namespace MQTTClient
{
    internal class Program
    {
        private static IMqttClient client;
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            client = new MqttClientService().MqttClientStart("127.0.0.1", 1883, "cyw", "123456", "cyw");
            client.ConnectedAsync += Client_ConnectedAsync;
            client.ApplicationMessageReceivedAsync += Client_ApplicationMessageReceivedAsync;
            client.DisconnectedAsync += Client_DisconnectedAsync;
            Console.ReadLine();
        }
        private static Task Client_DisconnectedAsync(MqttClientDisconnectedEventArgs arg)
        {
            Console.WriteLine("Client_DisconnectedAsync");
            return Task.CompletedTask;
        }
        private static Task Client_ApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs arg)
        {
            Console.WriteLine($"ApplicationMessageReceivedAsync：客户端ID=【{arg.ClientId}】接收到消息。 Topic主题=【{arg.ApplicationMessage.Topic}】 消息=【{Encoding.UTF8.GetString(arg.ApplicationMessage.Payload)}】 qos等级=【{arg.ApplicationMessage.QualityOfServiceLevel}】");
            return Task.CompletedTask;
        }
        private static System.Threading.Tasks.Task Client_ConnectedAsync(MqttClientConnectedEventArgs arg)
        {
            client.SubscribeAsync("test", MqttQualityOfServiceLevel.AtLeastOnce);
            Console.WriteLine("Client_ConnectedAsync");
            return Task.CompletedTask;
        }
    }
}