openssl ecparam -name prime256v1 -genkey -noout -out device.key

openssl req -new -key device.key -out device.csr -subj "/CN=device.example.com/O=YourOrg/OU=IoT"

 openssl x509 -req -in device.csr -CA IntermediateCA.crt -CAkey IntermediateCA.key -CAcreateserial -out device.crt -days 365 -sha256

The PEM encoded root CA certificate chain that is used to validate the server
public const string CA_ROOT_PEM = @"-----BEGIN CERTIFICATE-----
CN: CN = Microsoft Azure ECC TLS Issuing CA 03
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
CN: CN = DigiCert Global Root G3
-----END CERTIFICATE-----";

The PEM encoded certificate chain that is used to authenticate the device
public const string CLIENT_CERT_PEM_A = @"-----BEGIN CERTIFICATE-----
-----BEGIN CERTIFICATE-----
 CN=Self signed device certificate
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
 CN=Self signed Intermediate certificate
-----END CERTIFICATE-----";

 The PEM encoded private key of device
public const string CLIENT_KEY_PEM_A = @"-----BEGIN EC PRIVATE KEY-----
-----END EC PRIVATE KEY-----";

public static void Main()
{
   int sequenceNumber = 0;
   MqttClient mqttClient = null;
   Thread.Sleep(1000); // Found this works around some issues with running immediately after a reset

   bool wifiConnected = false;
   Console.WriteLine("WiFi connecting...");
   do
   {
      // Attempt to connect using DHCP
      wifiConnected = WifiNetworkHelper.ConnectDhcp(Secrets.WIFI_SSID, Secrets.WIFI_PASSWORD, requiresDateTime: true);

      if (!wifiConnected)
      {
         Console.WriteLine($"Failed to connect. Error: {WifiNetworkHelper.Status}");
         if (WifiNetworkHelper.HelperException != null)
         {
            Console.WriteLine($"Exception: {WifiNetworkHelper.HelperException}");
         }

         Thread.Sleep(1000);
      }
   }
   while (!wifiConnected);
   Console.WriteLine("WiFi connected");

   var caCert = new X509Certificate(Constants.CA_ROOT_PEM);

   X509Certificate2 clientCert = null;
   try
   {
      clientCert = new X509Certificate2(Secrets.CLIENT_CERT_PEM_A, Secrets.CLIENT_KEY_PEM_A, string.Empty);
   }
   catch (Exception ex)
   {
      Console.WriteLine($"Client Certificate Exception: {ex.Message}");
   }

   mqttClient = new MqttClient(Secrets.MQTT_SERVER, Constants.MQTT_PORT, true, caCert, clientCert, MqttSslProtocols.TLSv1_2);

   mqttClient.ProtocolVersion = MqttProtocolVersion.Version_5;

   bool mqttConnected = false;
   Console.WriteLine("MQTT connecting...");
   do
   {
      try
      {
         // Regular connect
         var resultConnect = mqttClient.Connect(Secrets.MQTT_CLIENTID, Secrets.MQTT_USERNAME, Secrets.MQTT_PASSWORD);
         if (resultConnect != MqttReasonCode.Success)
         {
            Console.WriteLine($"MQTT ERROR connecting: {resultConnect}");
            Thread.Sleep(1000);
         }
         else
         {
            mqttConnected = true;
         }
      }
      catch (Exception ex)
      {
         Console.WriteLine($"MQTT ERROR Exception '{ex.Message}'");
         Thread.Sleep(1000);
      }
   }
   while (!mqttConnected);
   Console.WriteLine("MQTT connected...");

   mqttClient.MqttMsgPublishReceived += MqttMsgPublishReceived;
   mqttClient.MqttMsgSubscribed += MqttMsgSubscribed;
   mqttClient.MqttMsgUnsubscribed += MqttMsgUnsubscribed;
   mqttClient.ConnectionOpened += ConnectionOpened;
   mqttClient.ConnectionClosed += ConnectionClosed;
   mqttClient.ConnectionClosedRequest += ConnectionClosedRequest;

   string topicPublish = string.Format(MQTT_TOPIC_PUBLISH_FORMAT, Secrets.MQTT_CLIENTID);
   while (true)
   {
      Console.WriteLine("MQTT publish message start...");

      var payload = new MessagePayload() { ClientID = Secrets.MQTT_CLIENTID, Sequence = sequenceNumber++ };

      string jsonPayload = JsonSerializer.SerializeObject(payload);

      var result = mqttClient.Publish(topicPublish, Encoding.UTF8.GetBytes(jsonPayload), "application/json; charset=utf-8", null);

      Debug.WriteLine($"MQTT published ({result}): {jsonPayload}");

      Thread.Sleep(100);
   }
}