Action Extensions

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Diagnostics;

static class ActionExtensions
{
  class WaitList : IDisposable
  {
    readonly List<IAsyncResult> waits = new List<IAsyncResult>();

    public void Add(IAsyncResult ar)
    {
      waits.Add(ar);
    }

    public void Dispose()
    {
      foreach (var ar in waits)
      {
        ar.AsyncWaitHandle.WaitOne();
      }
    }
  }

  public static Action MakeParallel(this Action t)
  {
    return () =>
    {
      using (var w = new WaitList())
      {
        foreach (Action a in t.GetInvocationList())
        {
          w.Add(a.BeginInvoke(null, null));
        }
      }
    };
  }

  public static Action<T1> MakeParallel<T1>(this Action<T1> t)
  {
    return (x) =>
    {
      using (var w = new WaitList())
      {
        foreach (Action<T1> a in t.GetInvocationList())
        {
          w.Add(a.BeginInvoke(x, null, null));
        }
      }
    };
  }

  public static Action<T1, T2> MakeParallel<T1, T2>(this Action<T1, T2> t)
  {
    return (x, y) =>
    {
      using (var w = new WaitList())
      {
        foreach (Action<T1, T2> a in t.GetInvocationList())
        {
          w.Add(a.BeginInvoke(x, y, null, null));
        }
      }
    };
  }

  public static Action<T1, T2, T3> MakeParallel<T1, T2, T3>(this Action<T1, T2, T3> t)
  {
    return (x, y, z) =>
    {
      using (var w = new WaitList())
      {
        foreach (Action<T1, T2, T3> a in t.GetInvocationList())
        {
          w.Add(a.BeginInvoke(x, y, z, null, null));
        }
      }
    };
  }

  public static Action<T1, T2, T3, T4> MakeParallel<T1, T2, T3, T4>(this Action<T1, T2, T3, T4> t)
  {
    return (x, y, z, v) =>
    {
      using (var w = new WaitList())
      {
        foreach (Action<T1, T2, T3, T4> a in t.GetInvocationList())
        {
          w.Add(a.BeginInvoke(x, y, z, v, null, null));
        }
      }
    };
  }
}

class Program
{
  static void Main(string[] args)
  {
    Action<int> f = null;

    for (int i = 0; i < 8; i++)
    {
      f += Thread.Sleep;  
    }
    
    Stopwatch ws = Stopwatch.StartNew();

    f(250);

    Console.WriteLine("ser: {0:f3}", ws.Elapsed.TotalMilliseconds);

    f = f.MakeParallel();

    ws = Stopwatch.StartNew();

    f(250);

    Console.WriteLine("par: {0:f3}", ws.Elapsed.TotalMilliseconds);

    Console.ReadLine();
  }
}