Using MQTTNet, how to subscribe to topic, parse then push back in a console application all together

Using MQTTNet, I am not very familiar with async in C# so I need a little bit help.

I want to subscribe to a MQTT topic (external broker like mosquitto), parse messages, and for particular ones push something back. Pretty simple and already working in my pieces

This is my subscribe

public static async Task Subscribe_Topic() 
{
    /*
     * This sample subscribes to a topic.
     */

    var mqttFactory = new MqttFactory();

    var mqttClient = mqttFactory.CreateMqttClient();

    mqttClient.ApplicationMessageReceivedAsync += e =>
        {
            await ConnectAndSendMessage(); // NOT working

            Console.WriteLine("Received application message.");
            Console.WriteLine(e.ClientId);
            return Task.CompletedTask;
        };

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

    var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
        .WithTopicFilter(
            f =>
            {
                f.WithTopic("my_topic");
            })
        .Build();

    var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);

    Console.WriteLine("MQTT client subscribed to topic ");

    // The response contains additional data sent by the server after subscribing.
    //response.DumpToConsole();
    //}
} 

Which works, using this ugly while() I came up with

static async Task Main(string[] args)
{

    await Subscribe_Topic();


    while (true) ;


}

This is what I use for connect

   public static MqttClientOptions mqttClientOptions = new MqttClientOptionsBuilder()
.WithTcpServer("10.77.150.243", 8883)
        .WithTimeout(new TimeSpan(600000000))
.WithCredentials("mydevice1", "mypass")
.WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V311)
.WithClientId("MY_ID")
.WithTls(new MqttClientOptionsBuilderTlsParameters()
{
    AllowUntrustedCertificates = true,
    SslProtocol = System.Security.Authentication.SslProtocols.Tls12,
    IgnoreCertificateChainErrors = true,
    UseTls = true,
})
.Build();

This my PUB function

    public static async Task ConnectAndSendMessage()
    {
        var mqttFactory = new MqttFactory();

        using (var mqttClient = mqttFactory.CreateMqttClient())
        {

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


            string tmp = @"xxntrary to popular belief, Lorem Ipsum ...";

            var applicationMessage = new MqttApplicationMessageBuilder()
                .WithTopic("my_other_topic")
                .WithRetainFlag(false)
                .WithQualityOfServiceLevel(MQTTnet.Protocol.MqttQualityOfServiceLevel.ExactlyOnce)
                .WithPayload(tmp)
                .Build();

            await mqttClient.PublishAsync(applicationMessage, CancellationToken.None);

            await mqttClient.DisconnectAsync();

            Console.WriteLine("MQTT application message is published.");
        }

    }

Now how can I send if I receive certain message?

What I can do is to implement a background worker instead of that while() but I think I already have async stuff here, so may be not needed.

Just don't know how to connect them properly.

Thanks for hints!

In order for the line of code you provided, await ConnectAndSendMessage();, to work correctly, it needs to be within an async function. To address this, I recommend modifying the code as shown below:

mqttClient.ApplicationMessageReceivedAsync += async e =>
{
    await ConnectAndSendMessage(); // only work for async function
    Console.WriteLine("Received application message.");
    Console.WriteLine(e.ClientId);
    return; // You can either use 'return;' or simply omit the return statement.
};

You can make the anonymous function asynchronous by adding the async keyword before the argument e. Regarding the return statement, you have the option to use return; or omit it entirely; both approaches are acceptable.