1.MQTTnet开源库，https://github.com/chkr1011/MQTTnet

MQTTnet.dll 2.8.5 下载

链接：https://pan.baidu.com/s/1BjQ41vOqQ6-JoY06m2uN4Q 
提取码：a18s 
（此版本示例：https://download.csdn.net/download/uaime/11359429 我尝试升级版本，很多方法不支持，没耐心看文档）

知识点（关于MQTT的详细说明 http://x5zj.com/?p=42）

2.服务器端和客户端（看上实例下载）

服务器端和客户端两个，他们需要保持长连接，主要是通过订阅和发布来进行消息的传递交换。

MQTT 服务端主要用于与多个客户端保持连接，并处理客户端的发布和订阅等逻辑。一般很少直接从服务端发送消息给客户端（可以使用 mqttServer.Publish(appMsg); 直接发送消息），多数情况下服务端都是转发主题匹配的客户端消息，在系统中起到一个中介的作用。

而客户端主要是通过向服务端订阅它感兴趣（主题）的消息，另一些客户端向服务端发布（主题）消息，服务端将订阅和发布的主题进行匹配，并将消息转发给匹配通过的客户端。

3连接，通信

（1）服务器端启动服务：

 public static IMqttServer mqttServer = null; //创建对象

// 配置一个mqtt服务.
var optionsBuilder = new MqttServerOptionsBuilder()
//连接记录数，默认 一般为2000
.WithConnectionBacklog(2000)
//服务器连接端口
.WithDefaultEndpointPort(port)
//连接验证器
.WithConnectionValidator(ValidatingMqttClients())
//持续会话
.WithPersistentSessions()

;
// 建立一个MQTT服务.
mqttServer = new MqttFactory().CreateMqttServer();
mqttServer.ApplicationMessageReceived += MqttServer_ApplicationMessageReceived;
mqttServer.ClientSubscribedTopic += MqttServer_ClientSubscribedTopic;
mqttServer.ClientUnsubscribedTopic += MqttServer_ClientUnsubscribedTopic;
mqttServer.ClientConnected += MqttServer_ClientConnected;
mqttServer.ClientDisconnected += MqttServer_ClientDisconnected;

//启动mqtt服务
await Task.Run(async () => { await mqttServer.StartAsync(optionsBuilder.Build()); });

（2）客户端

与服务器端连接

连接语句：

 private IMqttClient mqttClient = null; //客户端对象

//实例化 创建客户端对象
var Factory = new MqttFactory();
mqttClient = Factory.CreateMqttClient();
mqttClient.ApplicationMessageReceived += MqttClient_ApplicationMessageReceived;
mqttClient.Connected += MqttClient_Connected;
mqttClient.Disconnected += MqttClient_Disconnected;

//调用异步方法连接到服务端

await mqttClient.ConnectAsync(option());

option的信息设置：

public IMqttClientOptions option()
{
//连接到服务器前，获取所需要的MqttClientTcpOptions 对象的信息
var options = new MqttClientOptionsBuilder()
.WithClientId(ClientId)                    // clientid是设备id
.WithTcpServer(IP, Port)              //onenet ip：183.230.40.39    port:6002
.WithCredentials(UserName,pwd)      //username为产品id       密码为鉴权信息或者APIkey
//.WithTls()//服务器端没有启用加密协议，这里用tls的会提示协议异常
.WithCleanSession(false)
.WithKeepAlivePeriod(TimeSpan.FromSeconds(2000))
.Build();
return options;
}

服务端支持 ClientConnected、ClientDisconnected 和 ApplicationMessageReceived 事件，分别用来检查客户端连接、客户端断开以及接收客户端发来的消息。

客户端支持 Connected、Disconnected 和 ApplicationMessageReceived 事件，用来处理客户端与服务端连接、客户端从服务端断开以及客户端收到消息的事情。

通过 mqtt接入onenet，其实也就是把onenet 平台当做mqtt broker，连接上稍作改变就可以了。

客户端（接收服务推送）封装类参考：

MQQT服务的源码，下载源码地址：https://github.com/mqtt/mqtt.github.io/wiki/libraries

public class MqttClientService
    {

        private static volatile MqttClientService _instance = null;

        private static readonly object LockHelper = new object();

        /// <summary>
        /// 创建单例模式
        /// </summary>
        /// <param name="ipAddress"></param>
        /// <returns></returns>
        public static MqttClientService CreateInstance(string ipAddress)
        {
            if (_instance == null)
            {
                lock (LockHelper)
                {
                    if (_instance == null)
                        _instance = new MqttClientService(ipAddress);
                }
            }
            return _instance;
        }

        /// <summary>
        /// 实例化订阅客户端
        /// </summary>
        public MqttClient SubscribeClient { get; set; }


        public Action<Object, MqttMsgPublishEventArgs> ClientPublishReceivedAction { get; set; }

        public MqttClientService(string ipAddress)
        {
            // create client instance 
            SubscribeClient = new MqttClient(IPAddress.Parse(ipAddress));

            // register to message received 
            SubscribeClient.MqttMsgPublishReceived += client_MqttMsgPublishReceived;

            string clientId = Guid.NewGuid().ToString();

            SubscribeClient.Connect(clientId);
            //订阅时修改路径（主题）即可
            // subscribe to the topic "/home/temperature" with QoS 2 
            SubscribeClient.Subscribe(new string[] { "avatar/uploaded" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE });
        }

        void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
        {
            // handle message received 
            ClientPublishReceivedAction.Invoke(sender, e);
        }

        public void client_MqttMsgPublish(string publishString)
        {
            SubscribeClient.Publish("avatar/signed", Encoding.UTF8.GetBytes(publishString), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, false);
        }
    }

C#mqtt库M2Mqtt的使用方法

目前最好用的C#库是 eclipse出的M2Mqtt库，

主页链接： http://www.eclipse.org/paho/clients/dotnet/

项目的地址是 https://github.com/eclipse/paho.mqtt.m2mqtt

使用方式是在vs 的命令中输入 Install-Package M2Mqtt
 

... 
 
// create client instance 
MqttClient client = new MqttClient(IPAddress.Parse(MQTT_BROKER_ADDRESS)); //注意是域名哦不支持IP
 
string clientId = Guid.NewGuid().ToString(); 
client.Connect(clientId); 
 
string strValue = Convert.ToString(value); 
 
// publish a message on "/home/temperature" topic with QoS 2 
client.Publish("/home/temperature", Encoding.UTF8.GetBytes(strValue), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, false); 
然而，却存在一个坑， MQTT_BROKER_ADDRESS 不能用IP地址，而要用域名，用IP地址会出现在Connect这一步连接失败。

放一个示例，可以直接使用的。

https://sdk.bce.baidu.com/console-sdk/mqtt-net.rar?responseContentDisposition=attachment

这个是百度云mqtt物接入的一个例子，用的是mqtt协议，下载后，把域名改一下，就可以用了。

mqtt服务器可以采用http://mosquitto.org/download/ 实现的mosquitto服务器。

Source

• mosquitto-1.6.3.tar.gz (319kB) (GPG signature)
• Git source code repository (github.com)

Older downloads are available at https://mosquitto.org/files/

Windows

• mosquitto-1.6.3-install-windows-x64.exe (~1.4 MB) (64-bit build, Windows Vista and up, built with Visual Studio Community 2017)
• mosquitto-1.6.3-install-windows-x32.exe (~1.4 MB) (32-bit build, Windows Vista and up, built with Visual Studio Community 2017)

组件下载：M2Mqtt.Net.dll （v4.3.0最新版 2019.7.16本地生成）

链接：https://pan.baidu.com/s/1lacs13v9nde8d2wrj2oU4g 
提取码：l8cg 

相关代码摘要：

//创建客户端实例（两种方式选择，否则出错，域名需转换）
MqttClient client = new MqttClient(IPAddress.Parse(MQTT_BROKER_ADDRESS)); //主机为IP时
MqttClient client = new MqttClient(MQTT_BROKER_ADDRESS); //当主机地址为域名时
 
// 注册消息接收处理事件，还可以注册消息订阅成功、取消订阅成功、与服务器断开等事件处理函数
client.MqttMsgPublishReceived += client_MqttMsgPublishReceived; 
 
//生成客户端ID并连接服务器
string clientId = Guid.NewGuid().ToString(); 
client.Connect(clientId); 
 
// 订阅主题"/home/temperature" 消息质量为 2 
// 三种参数：0 至多1次，1 至少1次，2 只有1次 （好像3保留）
client.Subscribe(new string[] { "/home/temperature" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE }); 
 
... 
 
void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e) 
{ 
//处理接收到的消息
string msg = System.Text.Encoding.Default.GetString(e.Message);
        textBox1.AppendText("收到消息:" + msg + "\r\n");
} 


//**************************************************************

// 发布消息到主题 "/home/temperature" 消息质量为 2,不保留 
client.Publish("/home/temperature", Encoding.UTF8.GetBytes("hello"), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, false); 

客户端代码参考2

服务端搭建：apache-apollo-***-windows，下载链接：http://activemq.apache.org/apollo/download.html

解压：在 apache-apollo    bin文件夹中，作如下操作：

输入：apollo.cmd create test 创建虚拟test服务器主机，在该文件夹下会生成一个test目录
进入 apache-apollo-1.7.1\bin\test\bin文件，开启服务器。
进入test目录下etc文件夹中，找到apollo.xml
打开浏览器：输入http://127.0.0.1:61680,任意一个都可以，即可进入管理的web界面，账户：admin ，密码：password
测试连接，打开mqtt用户客户端，查看服务器显示情况
 

using MQTTnet;
using MQTTnet.Core;
using MQTTnet.Core.Client;
using MQTTnet.Core.Packets;
using MQTTnet.Core.Protocol;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace mqtt_client
{
    public partial class Form1 : Form
    {
        private MqttClient mqttClient = null;
        public Form1()
        {
            InitializeComponent();
            Task.Run(async () => { await ConnectMqttServerAsync(); });
        }
        private async Task ConnectMqttServerAsync()
        {
            if (mqttClient == null)
            {
                mqttClient = new MqttClientFactory().CreateMqttClient() as MqttClient;
                mqttClient.ApplicationMessageReceived += MqttClient_ApplicationMessageReceived;
                mqttClient.Connected += MqttClient_Connected;
                mqttClient.Disconnected += MqttClient_Disconnected;
            }
 
            try
            {
                var options = new MqttClientTcpOptions
                {
                    Server = "127.0.0.1",
                    Port=61613,
                    ClientId = Guid.NewGuid().ToString(),
                    UserName = "admin",
                    Password = "password",
                    CleanSession = true
                };
 
                await mqttClient.ConnectAsync(options);
            }
            catch (Exception ex)
            {
                Invoke((new Action(() =>
                {
                    textBox2.AppendText($"连接到MQTT服务器失败！" + Environment.NewLine + ex.Message + Environment.NewLine);
                })));
            }
        }
        private void MqttClient_Connected(object sender, EventArgs e)
        {
            Invoke((new Action(() =>
            {
                textBox2.AppendText("已连接到MQTT服务器！" + Environment.NewLine);
            })));
        }
 
        private void MqttClient_Disconnected(object sender, EventArgs e)
        {
            Invoke((new Action(() =>
            {
                textBox2.AppendText("已断开MQTT连接！" + Environment.NewLine);
            })));
        }
 
        private void MqttClient_ApplicationMessageReceived(object sender, MqttApplicationMessageReceivedEventArgs e)
        {
            Invoke((new Action(() =>
            {
                textBox2.AppendText($">> {Encoding.UTF8.GetString(e.ApplicationMessage.Payload)}{Environment.NewLine}");
            })));
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            string topic = textBox1.Text.Trim();
 
            if (string.IsNullOrEmpty(topic))
            {
                MessageBox.Show("订阅主题不能为空！");
                return;
            }
 
            if (!mqttClient.IsConnected)
            {
                MessageBox.Show("MQTT客户端尚未连接！");
                return;
            }
 
            mqttClient.SubscribeAsync(new List<TopicFilter> {
                new TopicFilter(topic, MqttQualityOfServiceLevel.AtMostOnce)
            });
 
            textBox2.AppendText($"已订阅[{topic}]主题" + Environment.NewLine);
            textBox1.Enabled = false;
            button1.Enabled = false;
 
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
            string topic = textBox4.Text.Trim();
 
            if (string.IsNullOrEmpty(topic))
            {
                MessageBox.Show("发布主题不能为空！");
                return;
            }
 
            string inputString = textBox3.Text.Trim();
            var appMsg = new MqttApplicationMessage(topic, Encoding.UTF8.GetBytes(inputString), MqttQualityOfServiceLevel.AtMostOnce, false);
            mqttClient.PublishAsync(appMsg);
 
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            Sunisoft.IrisSkin.SkinEngine skin = new Sunisoft.IrisSkin.SkinEngine
            {
                SkinFile = @"./RealOne.ssk"
            };
        }
    }
}

参考资料：

https://github.com/mqtt/mqtt.github.io/wiki/libraries 

https://github.com/leytton/m2mqtt (此项目下载后依赖项很多出问题，尝试修改没弄成)

仅部分内容摘自网络整理而成，如有侵权请联系删除。