public async Task RemoveMessageSubscription(UnsubscribeMessageCommand command)
        {
            await Lock.WaitAsync();

            try
            {
                MessageSubscriptions.Remove(command.ReferenceId);
            }
            finally
            {
                Lock.Release();
            }
        }
        public async Task AddMessageSubscription(SubscribeMessageCommand command)
        {
            await Lock.WaitAsync();

            try
            {
                MessageSubscriptions.Add(command.ReferenceId, command.Topic);
            }
            finally
            {
                Lock.Release();
            }
        }