//Callback for MQTT subscription.
        async void IServiceCallback.MQTTSubscribe(String topic)
        {
            await mqttClientSemaphore.WaitAsync();

            try
            {
                if (m_topicSet.Contains(topic))
                {
                    return;
                }
                else
                {
                    ServiceLogic.ManagedMqttSubscribe(managedMqtt, topic);
                    m_topicSet.Add(topic);
                }
            }
            finally
            {
                mqttClientSemaphore.Release();
            }
        }