using System.Threading.Tasks;

using System;

using System.Collections.Generic;

​

await Process(Ints(), 10);

​

static IEnumerable<int> Ints(){

    for(var i = 0; i < 100; i++){

        yield return i;

    }

}

​

async Task DoThingAsync(int i){

    Console.WriteLine($"Starting {i}");

    await Task.Delay(20 + i + (i%5 * 100));

    Console.WriteLine($"Done {i}");

}

​

async Task Process(IEnumerable<int> ints, int maxDegreeOfParallellism){

    var currentTasks = new List<Task>();

    foreach(var n in ints){

        currentTasks.Add(DoThingAsync(n));

        if(currentTasks.Count < maxDegreeOfParallellism){

            continue;   

        }

        var completedTask = await Task.WhenAny(currentTasks);

        currentTasks.Remove(completedTask);

    }

    await Task.WhenAll(currentTasks);

}

​