using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading;

using System.Threading.Tasks;

public class Program

{

    private static Dictionary<int,IReadOnlyList<int>> resource = new();

    private static SemaphoreSlim semaphore = new(1);

    public static async Task Main()

    {       

        var outerLoopSource = Enumerable.Range(0,10);

        await Parallel.ForEachAsync(outerLoopSource, OuterLoopFunction);

        foreach(var (key, list) in resource)

            Console.WriteLine(key+": "+string.Join(',', list));

    }

    public static async ValueTask OuterLoopFunction(int i, CancellationToken cancel) {

        // some time consuming calculation ...      

        var key = i%3;

        const int listSize = 10;

        IReadOnlyList<int> list;

        await semaphore.WaitAsync();

        try {

            if(!resource.TryGetValue(key, out list)) {

                var newList = new int[listSize];

                list = newList;

                resource.Add(key, list);

                await Parallel.ForEachAsync(Enumerable.Range(0,listSize), InnerLoopFunction);

                ValueTask InnerLoopFunction(int j, CancellationToken cancel) {

                    // some time consuming calculation ...

                    newList[j] = 42+i;

                    return ValueTask.CompletedTask;

                }

            }

        } finally {

            semaphore.Release();

        }           

        // do something with list

    }   

}