1. 介绍

MQTTnet 是一个强大的开源 MQTT 客户端库，适用于 C# 平台。它提供了丰富的功能和灵活的 API，可以轻松地构建基于 MQTT 协议的应用程序。本文将逐步学习如何使用 MQTTnet 库创建 MQTT 客户端，并实现基本的发布、订阅功能。

简介

什么是MQTT?

MQTT的发布和订阅架构

2. 搭建MQTT Broker

在Windows 系统上搭建 MQTT Broker，前一篇文章 Docker部署Eclipse Mosquitto开源MQTT的消息代理环境步骤（实践），我们使用docker搭建了Mosquitto环境。如果你觉得比较麻烦，也可以使用软件包来搭建环境。

具体的搭建步骤，可以查看官网。

3. 安装MQTTnet库

通过 NuGet 包管理器将 MQTTnet 库添加到我们的项目中。

4. 项目编码开发

1. MqttSubscriber项目

1. 新建MqttSubscriber项目。
   
2. 配置引用MQTTnet库

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
	<ItemGroup>
		<PackageReference Include="MQTTnet" Version="4.3.3.952" />
		<PackageReference Include="MQTTnet.Extensions.ManagedClient" Version="4.3.3.952" />
	</ItemGroup>

</Project>

3. 编写订阅代码

using System.Text;
using MQTTnet;
using MQTTnet.Client;

class Program
{
    static async Task Main(string[] args)
    {
        var options = new MqttClientOptionsBuilder()
            .WithTcpServer("localhost", 1883)
            .WithCredentials("root","root")
            .Build();

        var factory = new MqttFactory();
        var mqttClient = factory.CreateMqttClient();

        mqttClient.ApplicationMessageReceivedAsync += MqttClient_ApplicationMessageReceivedAsync;

        await mqttClient.ConnectAsync(options);
        await mqttClient.SubscribeAsync("testtopic");

        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();

        await mqttClient.DisconnectAsync();
    }

    private static Task MqttClient_ApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs arg)
    {
        Console.WriteLine($"Received message: {Encoding.UTF8.GetString(arg.ApplicationMessage.PayloadSegment)}");
        return Task.CompletedTask;
    }
}

2. MqttPublisher项目

1. 新建MqttPublisher项目。
   
2. 配置引用MQTTnet库

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
	<ItemGroup>
		<PackageReference Include="MQTTnet" Version="4.3.3.952" />
		<PackageReference Include="MQTTnet.Extensions.ManagedClient" Version="4.3.3.952" />
	</ItemGroup>

</Project>

3. 编写发布代码

using System;
using System.Text;
using System.Threading.Tasks;
using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Extensions.ManagedClient;

class Program
{
    static async Task Main(string[] args)
    {
        var options = new MqttClientOptionsBuilder()
            .WithTcpServer("localhost", 1883)
            .WithCredentials("root", "root")
            .Build();

        var factory = new MqttFactory();
        var mqttClient = factory.CreateMqttClient();

        var managedMqttClient = new MqttFactory().CreateManagedMqttClient();
        await managedMqttClient.SubscribeAsync("testtopic");

        await mqttClient.ConnectAsync(options);
        bool bcontinue = true;
        while (bcontinue)
        {
            string input = $"{DateTime.Now.ToString("HH:mm:ss fff")}:{Guid.NewGuid().ToString()}";           
            Console.WriteLine(input);
            var message = new MqttApplicationMessageBuilder()
                .WithTopic("testtopic")
                .WithPayload(Encoding.UTF8.GetBytes(input))
                .WithQualityOfServiceLevel(MQTTnet.Protocol.MqttQualityOfServiceLevel.ExactlyOnce)
                .WithRetainFlag()
                .Build();

            await mqttClient.PublishAsync(message);
        }

        await managedMqttClient.StopAsync();
    }
}

4. 调试运行
   

5. 参考文档

• MQTT - The Standard for IoT Messaging
• Getting started
• 在.NET7中使用MQTTnet简单实现MQTT通信 - 翔星 - 博客园