using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

using System.Threading;

public class Program

{

    public static void Main()

    {

         List<SimpleClass> seq = Enumerable.Range(0, 25).Select(x => new SimpleClass(){ OwnNumber = x, ActualNumber = x }).ToList();

​

        for (var i = 0; i < seq.Count(); i++)

        {

            var res = seq.Test();

            foreach (var item in res)

            {

                Console.WriteLine($"{item.OwnNumber} : {item.ActualNumber}");

            }

        }

    }

}

​

public class SimpleClass

{

    public int OwnNumber { get; set; }

    public int ActualNumber { get; set; }

}

​

public static class SimpleClassHelper

{

    private static SimpleClass _sharedSimpleClassHelper;

    public static int Apply()

    {

        int z = 1;

        for (var i = 0; i < 1000; i++)

        {

            z += i;

        }

        return _sharedSimpleClassHelper.ActualNumber;

    }

​

    public static IEnumerable<SimpleClass> Test(this IEnumerable<SimpleClass> sc) =>

        sc.AsParallel().Select(s => 

        {

            _sharedSimpleClassHelper = s;

            return new SimpleClass

            {

                OwnNumber = s.OwnNumber,

                ActualNumber = Apply()

            };

        });

}