using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        await Task.WhenAll(Stuff(5));

    }

​

    public static IEnumerable<Task> Stuff(int i)

    {

        for (int x = 0; x < i; x++)

        {

            yield return Task.WhenAll(GetTasks(5));

        }

    }

​

    public static IEnumerable<Task<int>> GetTasks(int i)

        => Enumerable.Range(0, i).Select(x => Task.FromResult(i));

}