public Task ChangeStatus(long requestId, int currentStatusId, int newStatusId, string message)
        {
            var item = PendingRequests.First(it => it.Id == requestId);

            if (item.StatusId != currentStatusId)
            {
                var msg = string.Format("StatusId mismatch (expected: {0}, actual: {1})", currentStatusId, item.StatusId);
                throw new InvalidOperationException(msg);
            }

            item.StatusId = newStatusId;
            return(Task.CompletedTask);
        }