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public partial class Form1 : Form
{
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // create client instance

            MqttClient client = new MqttClient("10.14.9.205");

            // register to message received

            client.MqttMsgPublishReceived += client_MqttMsgPublishReceived;

            string clientId = Guid.NewGuid().ToString();

            client.Connect(clientId);

            // subscribe to the topic "/home/temperature" with QoS 2

            client.Subscribe(new string[] { "/home/temperature" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE });

            label1.Text = clientId;
        }

        static void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
        {
            // handle message received
        }
    }
}                    

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public partial class MainWindow : Window
    {
        MqttClient client;
        string clientId;

        public MainWindow()
        {
            InitializeComponent();

            string BrokerAddress = "10.14.9.205";

            client = new MqttClient(BrokerAddress);

            // register a callback-function (we have to implement, see below) which is called by the library when a message was received
            client.MqttMsgPublishReceived += client_MqttMsgPublishReceived;

            // use a unique id as client id, each time we start the application
            clientId = Guid.NewGuid().ToString();

            client.Connect(clientId);
            //txtTopicSubscribe.Text = clientId;

        }
        // this code runs when a message was received
        void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
        {
            string ReceivedMessage = Encoding.UTF8.GetString(e.Message);

            Dispatcher.Invoke(delegate {              // we need this construction because the receiving code in the library and the UI with textbox run on different threads
                txtReceived.Text = ReceivedMessage;
                
            });

        }
        protected override void OnClosed(EventArgs e)

        {
            client.Disconnect();
            base.OnClosed(e);
            App.Current.Shutdown();
        }
        

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (txtTopicSubscribe.Text != "")
            {
                // whole topic

                string Topic = "/ElektorMyJourneyIoT/" + txtTopicSubscribe.Text + "/test";

                // subscribe to the topic with QoS 2
                client.Subscribe(new string[] { Topic }, new byte[] { 2 });      // we need arrays as parameters because we can subscribe to different topics with one call
                txtReceived.Text = clientId;
               
            }
            else
            {
                System.Windows.MessageBox.Show("You have to enter a topic to subscribe!");
            }          
        }
    }
}