using System;

using System.Collections.Generic;

using System.Linq;

using System.Net;

using System.Net.NetworkInformation;

using System.Threading;

using System.Threading.Tasks;

using System.Threading.Tasks.Dataflow;

​

public class Program

{

    private static readonly SemaphoreSlim _sem = new SemaphoreSlim(3, 3);

    private static readonly Random _r = new Random();

    private static long _counter = 0;

    public static IPAddress RandomIpAddress()

    {

        var data = new byte[4];

        _r.NextBytes(data);

        return new IPAddress(data);

    }

​

    public static async Task<PingReply> SomethingAsync(IPAddress address)

    {

        try

        {

            Interlocked.Increment(ref _counter);

            var ping = new Ping();

            var result = await ping.SendPingAsync(address, 1000);

            Console.WriteLine($"{Interlocked.Read(ref _counter)} : {address} - {result.Status} {result.RoundtripTime}ms");

            return result;

        }

        catch (PingException ex)

        {

            Console.WriteLine($"{Interlocked.Read(ref _counter)} : {address} - {ex.Message}");

        }

        finally

        {

            Interlocked.Decrement(ref _counter);

        }

​

        return null;

    }

​

    public static async Task Test1(IEnumerable<IPAddress> list)

    {

        static async Task<PingReply> Process(IPAddress address)

        {

            try

            {

                await _sem.WaitAsync();

                return await SomethingAsync(address);

            }

            finally

            {

                _sem.Release();

            }

        }

​

        await Task.WhenAll(list.Select(Process));

    }

​

    public static async Task Test2(IEnumerable<IPAddress> list)

    {

        var block = new ActionBlock<IPAddress>(SomethingAsync, new ExecutionDataflowBlockOptions(){MaxDegreeOfParallelism = 3});

        await Task.WhenAll(list.Select(x => block.SendAsync(x)));

        block.Complete();

        await block.Completion;

    }

​

    static async Task Main()

    {

        // a list of something

        var list = Enumerable.Range(0, 12).Select(x => RandomIpAddress()).ToList();

        await Test1(list);

        Console.WriteLine("---------");

        await Test2(list);

    }

}