How do you print MQTT messages to the console?

Question

I have a very simple C# command line app that connects to an MQTT server and prints messages to the console.

using MQTTnet;
using MQTTnet.Client.Options;
using MQTTnet.Extensions.ManagedClient;
using System.Text;

var options = new MqttClientOptionsBuilder()
    .WithTcpServer(MqttConfig.Server, MqttConfig.Port)
    .WithCredentials(MqttConfig.User, MqttConfig.Password)
    .WithClientId("MqttTest")
    .WithCleanSession()
    .Build();

var MqttClient = new MqttFactory().CreateMqttClient();

var cancellationToken = new CancellationToken();

var subscribeOptions = new MQTTnet.Client.Subscribing.MqttClientSubscribeOptions();
subscribeOptions.TopicFilters.Add(new MqttTopicFilter { Topic = MqttConfig.Topic });

MqttClient.ConnectAsync(options, cancellationToken);

MqttClient.SubscribeAsync(subscribeOptions, cancellationToken);

MqttClient.UseApplicationMessageReceivedHandler(e => { HandleMessageReceived(e.ApplicationMessage); });

while (true)
{
    Task.Delay(1000).GetAwaiter().GetResult();
}

static void HandleMessageReceived(MqttApplicationMessage applicationMessage)
{
    Console.WriteLine("### RECEIVED MESSAGE ###");
    Console.WriteLine($"+ Topic = {applicationMessage.Topic}");
    Console.WriteLine($"+ Payload = {Encoding.UTF8.GetString(applicationMessage.Payload)}");
    Console.WriteLine();
}

abstract class MqttConfig
{
    public static readonly string Server   = "servername";
    public static readonly int    Port     = 1883;
    public static readonly string User     = "user";
    public static readonly string Password = "password";
    public static readonly string Topic    = "#";
}

Putting the MqttConfig class information into an app like MQTT X shows a bunch of incoming messages. But running this C# app just shows a blank console.

Answer 1

I ended up basing the application on an MQTTnet sample. I'm posting it as an answer here in case anyone else has the same question in the future.

using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Client.Options;
using System.Text.Json;

#region Subscribe to topic & handle incoming messages
var mqttFactory = new MqttFactory();

using (var mqttClient = mqttFactory.CreateMqttClient())
{
    var mqttClientOptions = new MqttClientOptionsBuilder()
        .WithTcpServer(MqttConfig.Server, MqttConfig.Port)
        .WithCredentials(MqttConfig.User, MqttConfig.Password)
        .Build();

    mqttClient.UseApplicationMessageReceivedHandler(e =>
    {
        Console.WriteLine("Received application message.");
        e.DumpToConsole();
        return Task.CompletedTask;
    });

    await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);

    var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
        .WithTopicFilter(f => f.WithTopic(MqttConfig.Topic))
        .Build();

    await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);

    Console.WriteLine("MQTT client subscribed to topic.");
    Console.ReadLine(); // Prevents app from immediately closing once MQTT subscription is complete.
                        // Will close if user presses "enter" before any messages are received.
}

static class ObjectExtensions
{
    public static TObject DumpToConsole<TObject>(this TObject @object)
    {
        var output = "NULL";
        if (@object != null)
        {
            output = JsonSerializer.Serialize(@object, new JsonSerializerOptions { WriteIndented = true });
        }

        Console.WriteLine($"[{@object?.GetType().Name}]:\r\n{output}");
        return @object;
    }
}
#endregion

static class MqttConfig
{
    public static readonly string Server   = "servername";
    public static readonly int    Port     = 1883;
    public static readonly string User     = "user";
    public static readonly string Password = "password";
    public static readonly string Topic    = "#";
}