Trying to run 100 parallel tasks on an iteration doesn't work

Question

Well, I'm trying to run a task 100 times on each run (with paralellism) but I can't manage this to work.

I'm trying to bruteforce an API, for this the API allows me to concatenate as many IDS as possible (without exceeding the timeout)

// consts:
// idsNum = 1000
// maxTasks = 100

// We prepare the ids that we will process (in that case 100,000)
var ids = PrepareIds(i * idsNum * maxTasks, idsNum, maxTasks);

// This was my old approach (didn't work)
//var result = await Task.WhenAll(ids.AsParallel().Select(async x => (await client.GetItems(x)).ToArray()));

// This is my new approach (also this didn't worked...)
var items = new List<item[]>();
ids.AsParallel().Select(x => client.GetItems(x).GetAwaiter().GetResult()).ForAll(item =>
{
    //Console.WriteLine("processed!");
    items.Add(item.ToArray());
});

var result = items.ToArray();

As you can see I put Console.WriteLine("processed!"); statment, in order to check if anything worked... But I can't manage this to work.

Those are my other methods:

    private static IEnumerable<ulong[]> PrepareIds(int startingId, int idsNum = 1000, int maxTasks = 100)
    {
        for (int i = 0; i < maxTasks; i++)
            yield return Range((ulong)startingId, (ulong)(startingId + idsNum)).ToArray();
    }

And...

    public static async Task<IEnumerable<item>> GetItems(this HttpClient client, ulong[] ids, Action notFoundCallback = null)
    {
        var keys = PrepareDataInKeys(type, ids); // This prepares the data to be sent to the API server
        var content = new FormUrlEncodedContent(keys);

        content.Headers.ContentType =
            new MediaTypeHeaderValue("application/x-www-form-urlencoded") { CharSet = "UTF-8" };
        client.DefaultRequestHeaders.ExpectContinue = false;

        // We create a post request
        var response = await client.PostAsync(new Uri(FILE_URL), content);
        string contents = null;
        JObject jObject;
        try
        {
            contents = await response.Content.ReadAsStringAsync();
            jObject = JsonConvert.DeserializeObject<JObject>(contents);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
            Console.WriteLine(contents);
            return null;
        }

        // Then we read the items from the parsed JObject
        JArray items;
        try
        {
            items = jObject
               .GetValue("...")
               .ToObject<JObject>()
               .GetValue("...").ToObject<JArray>();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
            return null;
        }

        int notFoundItems = 0;
        int nonxxxItems = 0;
        int xxxItems = 0;

        var all = items.BuildEnumerable(notFoundCallback, item =>
        {
            if (item.Result != 1)
                ++notFoundItems;
            else if (item.id != 5)
                ++nonxxxItems;
            else
                ++xxxItems;
        });

        CrawledItems += ids.Length;

        NotFoundItems += notFoundItems;
        NonXXXItems += nonxxxItems;
        XXXItems += xxxItems;

        return all;
    }

    private static IEnumerable<item> BuildEnumerable(this JArray items, Action notFoundCallback, Action<item> callback = null)
    {
        foreach (var item in items)
        {
            item _item;

            try
            {
                _item = new item(item.ToObject<JObject>());
                callback?.Invoke(_item);
            }
            catch (Exception ex)
            {
                if (notFoundCallback == null)
                    Console.WriteLine(ex, Color.Red);
                else
                    notFoundCallback();

                continue;
            }

            yield return _item;
        }
    }

So as you can see I create 100 parallel post requests using an HttpClient. But I can't manage it to work.

So the thing that I want to achieve is to retrieve as many items as possible because I need to crawl +2,000,000,000 items.

But any breakpoint is triggered, neither any caption is updated on Console (I'm using Konsole project in order to print values at a fixed position on console), so any advice can be given there?