关于MQTT的调研

MQTT也可以理解成是一种消息队列。但是区别其它的消息队列，MQTT主要是针对低带宽高延迟的环境设计的，所以比较适合一些物联网的设备使用。相对来说也会比较轻量一点。MQTT也针对物联网领域的安全方便做了一些设计。

EMQX

在EMQX（https://cloud.emqx.com/console/）注册了一个免费的服务。
官方C#语言推荐用M2Mqtt来连接服务器。
用M2Mqtt来连接免费的测试服务器，能够正常成功。
示例教程：
https://docs.emqx.com/zh/cloud/latest/connect_to_deployments/c_sharp_sdk.html

但是用M2Mqtt来连接自己的服务端，一直连接不成功。

目前已经调研成功，主要是修改一下一行代码：
MqttClient client = new MqttClient(broker, port, true, MqttSslProtocols.TLSv1_2, null, null);

M2Mqtt

安装：

Install-Package M2Mqtt

示例代码：

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

string broker = "a88b8b81.ala.cn-hangzhou.emqxsl.cn";
int port = 8883;
string topic = "Csharp/mqtt";
string clientId = Guid.NewGuid().ToString();
string username = "你的用户名";
string password = "你的密码";
MqttClient client = ConnectMQTT(broker, port, clientId, username, password);
Subscribe(client, topic);
Publish(client, topic);

static MqttClient ConnectMQTT(string broker, int port, string clientId, string username, string password)
{
    MqttClient client = new MqttClient(broker, port, true, MqttSslProtocols.TLSv1_2, 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 Publish(MqttClient client, string topic)
{
    int msg_count = 0;
    while (true)
    {
        System.Threading.Thread.Sleep(1 * 1000);
        string msg = "messages: " + msg_count.ToString();
        client.Publish(topic, System.Text.Encoding.UTF8.GetBytes(msg));
        Console.WriteLine("Send `{0}` to topic `{1}`", msg, topic);
        msg_count++;
    }
}

static void Subscribe(MqttClient client, string topic)
{
    client.MqttMsgPublishReceived += client_MqttMsgPublishReceived;
    client.Subscribe(new string[] { topic }, new byte[] { MqttMsgBase.QOS_LEVEL_AT_MOST_ONCE });
}

static void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
{
    string payload = System.Text.Encoding.Default.GetString(e.Message);
    Console.WriteLine("Received `{0}` from `{1}` topic", payload, e.Topic.ToString());
}