AggregateUntil() example

AggregateUntilUsage.cs

public static List<Label> GetLabels()
{
    /*
     * use linq's lazy loading feature to enumerate an unlimited number of page requests. so i created AggregateUntil() to
     * take in a Func() which will inform when to stop iterating - in this case, we want to stop after the entries count
     * exceeds the totalNumEntries value, which Google Adwords returns from the first query
     */ 
    var labels = Enumerable
        .Range(0, Int32.MaxValue) //range generates a list of integers, from 1 to practically infinity
        .AggregateUntil(
            new LabelPage()
            {
                entries = new Label[] { },
                totalNumEntries = 0,
            },
            (accumulatedLabelPage, pageIndex) =>
            {
                /*
                 * also use select() and first() to mitigate rate limiting; setup 5 identical requests and while the
                 * returned page is null (meaning there was a caught exception), keep retrying.  after first() stops
                 * the evaluation, or you've attempted up to the max number of times to fetch, there will be one
                 * non-empty page result.  if there is no result at all, Single() will throw an error, which is what we
                 * want to happen
                 */
                var thisLabelsPage = Enumerable
                    .Range(0, pageMaxFetchAttempts)
                    .Select(pageAttempt =>
                    {
                        // this is the exponential backoff
                        Thread.Sleep(2500 * pageAttempt * pageAttempt);
                        try
                        {
                            var labelsAwql =
                                $"select LabelId,LabelName,LabelStatus limit {(pageIndex * pageSize)},{pageSize}";
                            var attemptedlabelsPage = Config.SupplyLabelService.query(labelsAwql);

                            return attemptedlabelsPage;
                        }
                        catch (Exception e)
                        {
                            return new LabelPage();
                        }
                    })
                    .First(page => page != null);

                return new LabelPage()
                {
                    entries = accumulatedLabelPage
                        .entries
                        .ToList()
                        .Concat(thisLabelsPage.entries)
                        .ToArray(),
                    totalNumEntries = thisLabelsPage.totalNumEntries,
                    totalNumEntriesSpecified = thisLabelsPage.totalNumEntriesSpecified,
                    PageType = thisLabelsPage.PageType,
                };
            },
            (accumulatedLabelPage) =>
            {
                var shouldStop = accumulatedLabelPage.entries.Length >= accumulatedLabelPage.totalNumEntries;
                return shouldStop;
            })
        .entries
        .ToList();

    return labels;
}