环境

VS：.NET Framework 4.8
NuGet：MQTTnet 4.2.1.781

背景

MCU加4G模组要测试MQTT通讯，和功能开发，为方便自己调试，写了个小工具做简单命令首发

MQTT连接

MCU肯定是作为客户端client，项目中已经搭建好了MQTT Broker 服务器。
第一步需要先连接自己的服务器。

//定义一个mqtt客户端句柄
public static IMqttClient _mqttClient = null;private void MqttConnectAsync()
{//若已连接，则断开原来的连接，防止占用资源if (_mqttClient != null && _mqttClient.IsConnected == true){_mqttClient.DisconnectAsync();}var optionsBuilder = new MqttClientOptionsBuilder().WithTcpServer("mqtt-test.com", 1883) //服务器IP或域名 服务器端口号.WithCredentials("", "") //凭据 忽略.WithClientId("test_client") //客户端ID 如果写死则相同ID的同时客户端连接服务器时，会挤掉其他客户端 | 这里也跟可以生成随机数转成字符串.WithCleanSession() //清理会话.WithTls(new MqttClientOptionsBuilderTlsParameters{UseTls = false});var clientOptions = optionsBuilder.Build();_mqttClient = new MqttFactory().CreateMqttClient();//创建客户端//连接成功的回调函数_mqttClient.ConnectedAsync += _mqttClient_ConnectedAsync;//连接断开的回调函数_mqttClient.DisconnectedAsync += _mqttClient_DisconnectedAsync;//数据接收的回调函数_mqttClient.ApplicationMessageReceivedAsync += _mqttClient_ApplicationMessageReceivedAsync;//执行异步方式连接_mqttClient.ConnectAsync(clientOptions);
}private Task _mqttClient_ConnectedAsync(MqttClientConnectedEventArgs arg)
{//连接成功一般做2件事：一、告知上层已连接成功，二、订阅Topic来接收数据string topic2 = "/C2S/" + IMEI + "/UP";string topic3 = "/C2S/Public";_mqttClient.SubscribeAsync(topic3, MqttQualityOfServiceLevel.AtMostOnce);return Task.CompletedTask;
}private Task _mqttClient_DisconnectedAsync(MqttClientDisconnectedEventArgs arg)
{//连接断开：告知上层连接已断开return Task.CompletedTask;
}

MQTT数据发送

void _mqttClient_Publish(byte[] cmd)
{string topic = "S2C/" + IMEI + "/DOWN";//MQTT数据发送 即 publish 发布，需要2个参数：一、发布主题，二、发布数据（有效载荷）//发布主题，可以根据自己的规则定义，比如以设备4G模组的IMEI号组成_mqttClient.PublishBinaryAsync(topic, cmd);
}

MQTT数据接收

private Task _mqttClient_ApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs arg)
{byte[] bytes = arg.ApplicationMessage.Payload;if (bytes[0] == 0x01&& bytes[1] == 0x98&& bytes[2] == 0x76){byte[] byte_tmp = new byte[bytes.Length - 9];for (int i = 5; i < bytes.Length - 4; i++){byte_tmp[i - 5] = bytes[i];}//数据定义中，上行数据添加固定头，其后跟字符串数据//每次接收时，判断固定头，提取后面的字符串数据进行打印textBox1.AppendText(System.Text.Encoding.UTF8.GetString(byte_tmp) + "\r\n");}else{textBox1.AppendText("收到非法消息\r\n");}return Task.CompletedTask;
}