class Program
{
   private static Model.ApplicationSettings _applicationSettings;
   private static HiveMQClient _client;
   private static bool _publisherBusy = false;

   static async Task Main()
   {
      Console.WriteLine($"{DateTime.UtcNow:yy-MM-dd HH:mm:ss} Hive MQ client starting");

      try
      {
         // load the app settings into configuration
         var configuration = new ConfigurationBuilder()
               .AddJsonFile("appsettings.json", false, true)
               .AddUserSecrets<Program>()
         .Build();

         _applicationSettings = configuration.GetSection("ApplicationSettings").Get<Model.ApplicationSettings>();

         var optionsBuilder = new HiveMQClientOptionsBuilder();

         optionsBuilder
            .WithClientId(_applicationSettings.ClientId)
            .WithBroker(_applicationSettings.Host)
            .WithPort(_applicationSettings.Port)
            .WithUserName(_applicationSettings.UserName)
            .WithCleanStart(_applicationSettings.CleanStart)
            .WithClientCertificate(_applicationSettings.ClientCertificateFileName, _applicationSettings.ClientCertificatePassword)
            .WithUseTls(true);

         using (_client = new HiveMQClient(optionsBuilder.Build()))
         {
            _client.OnMessageReceived += OnMessageReceived;

            var connectResult = await _client.ConnectAsync();
            if (connectResult.ReasonCode != ConnAckReasonCode.Success)
            {
               throw new Exception($"Failed to connect: {connectResult.ReasonString}");
            }

            Console.WriteLine($"Subscribed to Topic");
            foreach (string topic in _applicationSettings.SubscribeTopics.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
            {
               var subscribeResult = await _client.SubscribeAsync(topic, _applicationSettings.SubscribeQualityOfService);

               Console.WriteLine($" Topic:{topic} Result:{subscribeResult.Subscriptions[0].SubscribeReasonCode}");
            }
   }
//...
}

class Program
{
   private static Model.ApplicationSettings _applicationSettings;
   private static IMqttClient _client;
   private static bool _publisherBusy = false;

   static async Task Main()
   {
      Console.WriteLine($"{DateTime.UtcNow:yy-MM-dd HH:mm:ss} MQTTNet client starting");

      try
      {
         // load the app settings into configuration
         var configuration = new ConfigurationBuilder()
               .AddJsonFile("appsettings.json", false, true)
               .AddUserSecrets<Program>()
         .Build();

         _applicationSettings = configuration.GetSection("ApplicationSettings").Get<Model.ApplicationSettings>();

         var mqttFactory = new MqttFactory();

         using (_client = mqttFactory.CreateMqttClient())
         {
            // Certificate based authentication
            List<X509Certificate2> certificates = new List<X509Certificate2>
            {
               new X509Certificate2(_applicationSettings.ClientCertificateFileName, _applicationSettings.ClientCertificatePassword)
            };

            var tlsOptions = new MqttClientTlsOptionsBuilder()
                  .WithClientCertificates(certificates)
                  .WithSslProtocols(System.Security.Authentication.SslProtocols.Tls12)
                  .UseTls(true)
                  .Build();

            MqttClientOptions mqttClientOptions = new MqttClientOptionsBuilder()
                     .WithClientId(_applicationSettings.ClientId)
                     .WithTcpServer(_applicationSettings.Host, _applicationSettings.Port)
                     .WithCredentials(_applicationSettings.UserName, _applicationSettings.Password)
                     .WithCleanStart(_applicationSettings.CleanStart)
                     .WithTlsOptions(tlsOptions)
                     .Build();

            var connectResult = await _client.ConnectAsync(mqttClientOptions);
            if (connectResult.ResultCode != MqttClientConnectResultCode.Success)
            {
               throw new Exception($"Failed to connect: {connectResult.ReasonString}");
            }

            _client.ApplicationMessageReceivedAsync += OnApplicationMessageReceivedAsync;

            Console.WriteLine($"Subscribed to Topic");
            foreach (string topic in _applicationSettings.SubscribeTopics.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
            {
               var subscribeResult = await _client.SubscribeAsync(topic, _applicationSettings.SubscribeQualityOfService);

               Console.WriteLine($" {topic} Result:{subscribeResult.Items.First().ResultCode}");
            }
      }
//...
}