I have an error called "Options error in MQTTNet.Client"

Question

I have built this below and got this error

I have MQTTNET version 4.1.6.1152. Can somebody help me solve this?

using System;
using System.Text;
using System.Threading.Tasks;
using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Client.Options;
using MQTTnet.Client.Subscribing;

namespace OTtoIT
{
    class Program
    {
        static async Task Main(string[] args)
        {
            string brokerAddress = "0.0.0.0";  // Your MQTT broker IP address
            int brokerPort = 1883;  // Default MQTT port
            string topic = "#";  // Your MQTT topic

            // Create a new MQTT client
            var mqttClient = new MqttFactory().CreateMqttClient();

            // Create client options using the MqttClientOptionsBuilder
            var mqttClientOptions = new MqttClientOptionsBuilder()
                .WithClientId("YourClientId")
                .WithTcpServer(brokerAddress, brokerPort)
                .WithCleanSession()
                .Build();

            // Set up the handler for receiving messages
            mqttClient.ApplicationMessageReceivedAsync += e =>
            {
                var payload = Encoding.UTF8.GetString(e.ApplicationMessage.PayloadSegment.ToArray());
                Console.WriteLine($"Received message: {payload}");
                return Task.CompletedTask;
            };

            // Connect to the MQTT broker
            await mqttClient.ConnectAsync(mqttClientOptions);
            Console.WriteLine("Connected to the broker.");

            // Subscribe to the topic
            var subscribeOptions = new MqttClientSubscribeOptionsBuilder()
                .WithTopicFilter(f => { f.WithTopic(topic); })
                .Build();

            await mqttClient.SubscribeAsync(subscribeOptions);
            Console.WriteLine($"Subscribed to topic: {topic}");

            // Keep the application running to receive messages
            Console.WriteLine("Press any key to exit...");
            Console.ReadKey();

            // Disconnect the client
            await mqttClient.DisconnectAsync();
            Console.WriteLine("Disconnected from the broker.");
        }
    }
}

and the error is

The type or namespace name 'Options' does not exist in the namespace 'MQTTnet.Client' (are you missing an assembly reference?)
The type or namespace name 'Subscribing' does not exist in the namespace 'MQTTnet.Client' (are you missing an assembly reference?)

Answer 1

Based on my test, I find there are 3 build errors in your project.

First, please remove all your reference related to MQTTNET and install Nuget-package MQTTnet.

Second, please change MqttFactory() to MqttClientFactory().

 var mqttClient = new MqttClientFactory().CreateMqttClient();

Third, please use Extension Methods to show the output in ApplicationMessageReceivedAsync, because e.ApplicationMessage can not be accessed.

Extension method:(Comes from Github)

internal 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;
    }
}

Deletegate Usage:

 // Set up the handler for receiving messages
 mqttClient.ApplicationMessageReceivedAsync += e =>
 {
     Console.WriteLine("Received application message.");
     e.DumpToConsole();

     return Task.CompletedTask;
     
 };

Here is a completed code example without building error.

using MQTTnet;
using System.Text.Json;

namespace OtTtoIT11
{
    internal class Program
    {
        static async Task Main(string[] args)
        {
            string brokerAddress = "0.0.0.0";  // Your MQTT broker IP address
            int brokerPort = 1883;  // Default MQTT port
            string topic = "#";  // Your MQTT topic

            // Create a new MQTT client
            var mqttClient = new MqttClientFactory().CreateMqttClient();

            // Create client options using the MqttClientOptionsBuilder
            var mqttClientOptions = new MqttClientOptionsBuilder()
                .WithClientId("YourClientId")
                .WithTcpServer(brokerAddress, brokerPort)
                .WithCleanSession()
                .Build();

            // Set up the handler for receiving messages
            mqttClient.ApplicationMessageReceivedAsync += e =>
            {
                Console.WriteLine("Received application message.");
                e.DumpToConsole();

                return Task.CompletedTask;
                
            };

            // Connect to the MQTT broker
            await mqttClient.ConnectAsync(mqttClientOptions);
            Console.WriteLine("Connected to the broker.");

            // Subscribe to the topic
            var subscribeOptions = new MqttClientSubscribeOptionsBuilder()
                .WithTopicFilter(f => { f.WithTopic(topic); })
                .Build();

            await mqttClient.SubscribeAsync(subscribeOptions);
            Console.WriteLine($"Subscribed to topic: {topic}");

            // Keep the application running to receive messages
            Console.WriteLine("Press any key to exit...");
            Console.ReadKey();

            // Disconnect the client
            await mqttClient.DisconnectAsync();
            Console.WriteLine("Disconnected from the broker.");
        }
    }

    internal 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;
        }
    }
}