using System;

using System.Collections.Generic;

using System.Linq;

using System.Collections.Concurrent;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        var events = new List<string>();

        foreach(var i in Enumerable.Range(0, 500)) {

            events.Add(Guid.NewGuid().ToString());

        }

        var bag1 = new ConcurrentBag<string>();

        var bag2 = new ConcurrentBag<string>();

        var bag3 = new ConcurrentBag<Task<string>>();

        var po = new ParallelOptions {MaxDegreeOfParallelism = 50 };

        Parallel.ForEach(events, po, evt => {

            if(int.TryParse(evt.Substring(0, 1), out var _)) {

                bag1.Add(evt);

            }

        } );

        Console.WriteLine(bag1.Count);

​

            Parallel.ForEach(events, po, async evt => {

            if(int.TryParse(evt.Substring(0, 1), out var _)) {

                await Task.Yield();

                bag2.Add(evt);

            }

        } );

        Console.WriteLine(bag2.Count);

            Parallel.ForEach(events, po, evt => {

            if(int.TryParse(evt.Substring(0, 1), out var _)) {

                bag3.Add(GetValue(evt));

            }

        } );

        Console.WriteLine(bag3.Count);

        await Task.WhenAll(bag3.ToArray());

        Console.WriteLine(bag3.Select(b => b.Result).Count());

    }

    private static async Task<string> GetValue(string v) {

        await Task.Yield();

        return v;

    }

}