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一、什么是 MQTT ？

MQTT（Message Queuing Telemetry Transport，消息队列遥测传输）是 IBM 开发的一个即时通讯协议，有可能成为物联网的重要组成部分。MQTT 是基于二进制消息的发布/订阅编程模式的消息协议，如今已经成为 OASIS 规范，由于规范很简单，非常适合需要低功耗和网络带宽有限的 IoT 场景。
github上还发现了一个项目，可以直接看协议：MQTT协议中文版

二、MQTTnet

MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker). The implementation is based on the documentation from http://mqtt.org/.

MQTT 开源库还有 MqttDotNet、nMQTT、M2MQTT 等，不过这几个里面，目前star最多的还是MQTTnet（编辑时间2018.5.9）。

三、创建项目并导入类库

在解决方案在右键单击-选择“管理解决方案的 NuGet 程序包”-在“浏览”选项卡下面搜索 MQTTnet，为服务端项目和客户端项目都安装上 MQTTnet 库。示例中使用的是2.7.5.0版本，不同版本最低要求的.net版本或其它支持，在NuGet选中MQTTNet，右侧可以看到具体描述。

客户端简单Demo可以见官方文档：https://github.com/chkr1011/MQTTnet/wiki/Client
本文示例程序github：https://github.com/landbroken/MQTTLearning

评论有不知道怎么用示例程序的，简单解释一下。目录一开始建的有点问题，调试稍微麻烦一点，懒得改了。

• 1、clone或download项目
• 2、vs2015或以上版本打开MQTTLearning/MqttServerTest/MqttServerTest.sln
• 3、服务器端示例在SeverTest项目里面
• 4、客户端示例在ClientTest项目里面
  调试：
  方法1）是vs里两个项目设为同时启动；
  方法2）一端用生成的exe启动，一端在vs里用debug启动

一般可以直接打开的，万一vs有路径依赖问题：

• 1、如果项目路径依赖有问题，删掉重新添加一遍SeverTest.csproj和ClientTest.csproj
• 2、如果MQTTnet 库引用路径有问题，删掉从packages里面重新引用一遍，或者nuget里面添加一下

3.1 服务器

直接跑一下示例程序就知道了
下面简单解释一下主要部分。

3.1.1 初始化并启动服务器

首先，初始化并启动服务器

1、这里是异步启动，用的2.7.5版本的库

Task.Run(async () => { 
    await StartMqttServer_2_7_5(); });

2、配置设置
WithDefaultEndpointPort是设置使用的端口，协议里默认是用1883，不过调试我改成8222了。
WithConnectionValidator是用于连接验证，验证client id，用户名，密码什么的。示例没用数据库，随便写死了两个值。
还有其他配置选项，比如加密协议，可以在官方文档里看看，示例就是先简单能用。

// Configure MQTT server.
                var optionsBuilder = new MqttServerOptionsBuilder()
                    .WithConnectionBacklog(100)
                    .WithDefaultEndpointPort(8222)
                    .WithConnectionValidator(ValidatingMqttClients())
                    ;

3、添加事件触发
ApplicationMessageReceived 是服务器接收到消息时触发的事件，可用来响应特定消息。
ClientConnected 是客户端连接成功时触发的事件。
ClientDisconnected 是客户端断开连接时触发的事件。

mqttServer = new MqttFactory().CreateMqttServer();
                mqttServer.ApplicationMessageReceived += MqttServer_ApplicationMessageReceived;
                mqttServer.ClientConnected += MqttServer_ClientConnected;
                mqttServer.ClientDisconnected += MqttServer_ClientDisconnected;

异步启动mqtt服务器，实际项目可能要加一个启动失败处理

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

示例里面为了简单调试玩的，写了几个触发命令，在启动后，输入对于字符串即可触发。

3.1.2 消息发送

mqtt的消息包含topic和payload两部分。topic就是消息主题（类型），用于另外一端判断这个消息是干什么用的。payload就是实际想要发送的数据。
WithTopic给一个topic。
WithPayload给一个msg。
WithAtMostOnceQoS设置QoS，至多1次。也可以设为别的。
PublishAsync异步发送出去。

string topic = "topic/hello";
var message = new MqttApplicationMessageBuilder()
                .WithTopic(topic)
                .WithPayload(msg)
                .WithAtMostOnceQoS()
                .WithRetainFlag()
                .Build();
await mqttServer.PublishAsync(message);

3.1.3 完整源码

using MQTTnet;
using MQTTnet.Diagnostics;
using MQTTnet.Protocol;
using MQTTnet.Server;
using System;
using System.Collections.Generic;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace MqttServerTest
{ 

class Program
{ 

private static IMqttServer mqttServer = null;
private static List<string> connectedClientId = new List<string>();
static void Main(string[] args)
{ 

Task.Run(async () => { 
 await StartMqttServer_2_7_5(); });
// Write all trace messages to the console window.
MqttNetGlobalLogger.LogMessagePublished += MqttNetTrace_TraceMessagePublished;
//2.4.0版本
//MqttNetTrace.TraceMessagePublished += MqttNetTrace_TraceMessagePublished;
//new Thread(StartMqttServer).Start();
while (true)
{ 

if (mqttServer==null)
{ 

Console.WriteLine("Please await mqttServer.StartAsync()");
Thread.Sleep(1000);
continue;
}
var inputString = Console.ReadLine().ToLower().Trim();
if (inputString == "exit")
{ 

Task.Run(async () => { 
 await EndMqttServer_2_7_5(); });
Console.WriteLine("MQTT服务已停止！");
break;
}
else if (inputString == "clients")
{ 

var connectedClients = mqttServer.GetConnectedClientsAsync();
Console.WriteLine($"客户端标识：");
//2.4.0
//foreach (var item in mqttServer.GetConnectedClients())
//{ 

// Console.WriteLine($"客户端标识：{item.ClientId}，协议版本：{item.ProtocolVersion}");
//}
}
else if (inputString.StartsWith("hello:"))
{ 

string msg = inputString.Substring(6);
Topic_Hello(msg);
}
else if (inputString.StartsWith("control:"))
{ 

string msg = inputString.Substring(8);
Topic_Host_Control(msg);
}
else if (inputString.StartsWith("subscribe:"))
{ 

string msg = inputString.Substring(10);
Subscribe(msg);
}
else
{ 

Console.WriteLine($"命令[{inputString}]无效！");
}
Thread.Sleep(100);
}
}
private static void MqttServer_ClientConnected(object sender, MqttClientConnectedEventArgs e)
{ 

Console.WriteLine($"客户端[{e.Client.ClientId}]已连接，协议版本：{e.Client.ProtocolVersion}");
connectedClientId.Add(e.Client.ClientId);
}
private static void MqttServer_ClientDisconnected(object sender, MqttClientDisconnectedEventArgs e)
{ 

Console.WriteLine($"客户端[{e.Client.ClientId}]已断开连接！");
connectedClientId.Remove(e.Client.ClientId);
}
private static void MqttServer_ApplicationMessageReceived(object sender, MqttApplicationMessageReceivedEventArgs e)
{ 

Console.WriteLine("### RECEIVED APPLICATION MESSAGE ###");
Console.WriteLine($"客户端[{e.ClientId}]>>");
Console.WriteLine($"+ Topic = {e.ApplicationMessage.Topic}");
Console.WriteLine($"+ Payload = {Encoding.UTF8.GetString(e.ApplicationMessage.Payload)}");
Console.WriteLine($"+ QoS = {e.ApplicationMessage.QualityOfServiceLevel}");
Console.WriteLine($"+ Retain = {e.ApplicationMessage.Retain}");
Console.WriteLine();
}
private static void MqttNetTrace_TraceMessagePublished(object sender, MqttNetLogMessagePublishedEventArgs e)
{ 

var trace = $">> [{e.TraceMessage.Timestamp:O}] [{e.TraceMessage.ThreadId}] [{e.TraceMessage.Source}] [{e.TraceMessage.Level}]: {e.TraceMessage.Message}";
if (e.TraceMessage.Exception != null)
{ 

trace += Environment.NewLine + e.TraceMessage.Exception.ToString();
}
Console.WriteLine(trace);
}
#region 2.7.5
private static async Task StartMqttServer_2_7_5()
{ 

if (mqttServer == null)
{ 

// Configure MQTT server.
var optionsBuilder = new MqttServerOptionsBuilder()
.WithConnectionBacklog(100)
.WithDefaultEndpointPort(8222)
.WithConnectionValidator(ValidatingMqttClients())
;
// Start a MQTT server.
mqttServer = new MqttFactory().CreateMqttServer();
mqttServer.ApplicationMessageReceived += MqttServer_ApplicationMessageReceived;
mqttServer.ClientConnected += MqttServer_ClientConnected;
mqttServer.ClientDisconnected += MqttServer_ClientDisconnected;
Task.Run(async () => { 
 await mqttServer.StartAsync(optionsBuilder.Build()); });
//mqttServer.StartAsync(optionsBuilder.Build());
Console.WriteLine("MQTT服务启动成功！");
}
}
private static async Task EndMqttServer_2_7_5()
{ 

if (mqttServer!=null)
{ 

await mqttServer.StopAsync();
}
else
{ 

Console.WriteLine("mqttserver=null");
}
}
private static Action<MqttConnectionValidatorContext> ValidatingMqttClients()
{ 

// Setup client validator. 
var options =new MqttServerOptions();
options.ConnectionValidator = c =>
{ 

Dictionary<string, string> c_u = new Dictionary<string, string>();
c_u.Add("client001", "username001");
c_u.Add("client002", "username002");
Dictionary<string, string> u_psw = new Dictionary<string, string>();
u_psw.Add("username001", "psw001");
u_psw.Add("username002", "psw002");
if (c_u.ContainsKey(c.ClientId) && c_u[c.ClientId] == c.Username)
{ 

if (u_psw.ContainsKey(c.Username) && u_psw[c.Username] == c.Password)
{ 

c.ReturnCode = MqttConnectReturnCode.ConnectionAccepted;
}
else
{ 

c.ReturnCode = MqttConnectReturnCode.ConnectionRefusedBadUsernameOrPassword;
}
}
else
{ 

c.ReturnCode = MqttConnectReturnCode.ConnectionRefusedIdentifierRejected;
}
};
return options.ConnectionValidator;
}
private static void Usingcertificate(ref MqttServerOptions options)
{ 

var certificate = new X509Certificate(@"C:\certs\test\test.cer", "");
options.TlsEndpointOptions.Certificate = certificate.Export(X509ContentType.Cert);
var aes = new System.Security.Cryptography.AesManaged();
}
#endregion
#region Topic
private static async void Topic_Hello(string msg)
{ 

string topic = "topic/hello";
//2.4.0版本的
//var appMsg = new MqttApplicationMessage(topic, Encoding.UTF8.GetBytes(inputString), MqttQualityOfServiceLevel.AtMostOnce, false);
//mqttClient.PublishAsync(appMsg);
var message = new MqttApplicationMessageBuilder()
.WithTopic(topic)
.WithPayload(msg)
.WithAtMostOnceQoS()
.WithRetainFlag()
.Build();
await mqttServer.PublishAsync(message);
}
private static async void Topic_Host_Control(string msg)
{ 

string topic = "topic/host/control";
var message = new MqttApplicationMessageBuilder()
.WithTopic(topic)
.WithPayload(msg)
.WithAtMostOnceQoS()
.WithRetainFlag(false)
.Build();
await mqttServer.PublishAsync(message);
}
/// <summary>
/// 替指定的clientID订阅指定的内容
/// </summary>
/// <param name="topic"></param>
private static void Subscribe(string topic)
{ 

List<TopicFilter> topicFilter = new List<TopicFilter>();
topicFilter.Add(new TopicFilterBuilder()
.WithTopic(topic)
.WithAtMostOnceQoS()
.Build());
//给"client001"订阅了主题为topicFilter的payload
mqttServer.SubscribeAsync("client001", topicFilter);
Console.WriteLine($"Subscribe:[{"client001"}]，Topic：{topic}");
}
#endregion
}
}

3.2 客户端

未完待续

参考文献

1、《使用 MQTTnet 快速实现 MQTT 通信》：链接
这篇文章是vs2017+.net core+mqttnet2.4.0的，目前库已经更新了好几个版本，如果用最新版的不能直接运行文章里的程序。
2、MQTT官网
3、开源库地址：MQTTnet
4、开源库对应文档：https://github.com/chkr1011/MQTTnet/wiki/Client