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

            try
            {
                ServiceLogic.ManagedMqttUnsubscribe(managedMqtt, topic);
                m_topicSet.Remove(topic);
            }
            finally
            {
                mqttClientSemaphore.Release();
            }
        }