使用M2Mqtt连接公共MQTT Broker失败求助（.NET Core 3.1）

我尝试用C#连接多个公共MQTT Broker均失败。按照EMQX官方文档编写代码，基于.NET Core 3.1框架与M2Mqtt 4.3.0 NuGet包，出现错误提示：

A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

测试过的公共Broker包括broker.emqx.io、broker.hivemq.com、mqtt.eclipse.org、test.mosquitto.org，即使移除认证凭证仍无法连接。有没有开发者成功用C#连接过公共MQTT Broker？求解决方案或建议。

带凭证测试代码 #

using System;
using uPLibrary.Networking.M2Mqtt;
using uPLibrary.Networking.M2Mqtt.Messages;

namespace csharpMQTT
{
    class Program
    {
        static MqttClient ConnectMQTT(string broker, int port, string clientId, string username, string password)
        {
            MqttClient client = new MqttClient(broker, port, false, MqttSslProtocols.None, null, null);
            client.Connect(clientId, username, password);
            if (client.IsConnected)
            {
                Console.WriteLine("Connected to MQTT Broker");
            }
            else
            {
                Console.WriteLine("Failed to connect");
            }
            return client;
        }

        static void Main(string[] args)
        {
            string broker = "broker.emqx.io";
            int port = 1883;
            string clientId = Guid.NewGuid().ToString();
            string username = "emqx";
            string password = "public";
            MqttClient client = ConnectMQTT(broker, port, clientId, username, password);
        }
    }
}

无凭证测试代码 #

using System;
using uPLibrary.Networking.M2Mqtt;
using uPLibrary.Networking.M2Mqtt.Messages;

namespace csharpMQTT
{
    class Program
    {
        static MqttClient ConnectMQTT(string broker, int port, string clientId)
        {
            MqttClient client = new MqttClient(broker, port, false, MqttSslProtocols.None, null, null);
            client.Connect(clientId);
            if (client.IsConnected)
            {
                Console.WriteLine("Connected to MQTT Broker");
            }
            else
            {
                Console.WriteLine("Failed to connect");
            }
            return client;
        }

        static void Main(string[] args)
        {
            string broker = "broker.hivemq.com";
            int port = 1883;
            string clientId = Guid.NewGuid().ToString();
            MqttClient client = ConnectMQTT(broker, port, clientId);
        }
    }
}

1. 排查网络环境 #

• 先用ping broker.emqx.io或telnet broker.emqx.io 1883测试网络连通性，多数连接失败是因为防火墙、公司网络代理或运营商屏蔽了1883端口。
• 切换手机热点测试，排除本地网络限制；若公司网络拦截，尝试使用SSL端口（如8883）连接。

2. 更换MQTT客户端库 #

M2Mqtt是较老旧的库，对.NET Core兼容性有限。推荐使用MQTTnet——目前维护活跃、支持全.NET平台的主流MQTT客户端库。

MQTTnet示例代码 #

using System;
using System.Threading.Tasks;
using MQTTnet;
using MQTTnet.Client;

namespace csharpMQTT
{
    class Program
    {
        static async Task Main(string[] args)
        {
            var factory = new MqttFactory();
            var mqttClient = factory.CreateMqttClient();

            var options = new MqttClientOptionsBuilder()
                .WithTcpServer("broker.emqx.io", 1883)
                .WithCredentials("emqx", "public")
                .WithClientId(Guid.NewGuid().ToString())
                .Build();

            try
            {
                await mqttClient.ConnectAsync(options);
                Console.WriteLine("Connected to MQTT Broker");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"连接失败：{ex.Message}");
            }
        }
    }
}

使用前需通过NuGet安装MQTTnet包。

3. 调整M2Mqtt连接参数（若坚持使用原库） #

• 设置更长的连接超时：client.ConnectTimeout = 10000;（10秒），避免因默认超时过短导致失败。
• 尝试切换到SSL端口（如8883），修改代码中port参数并将MqttSslProtocols.None改为对应SSL协议（如MqttSslProtocols.TLSv1_2）。

4. 验证基础配置 #

• 确保客户端ID唯一（用Guid.NewGuid().ToString()生成是可靠的）。
• 确认认证信息正确：比如broker.emqx.io的默认凭证是emqx/public，无凭证Broker需调用无参Connect方法。

内容的提问来源于stack exchange，提问作者bags