4Threads.cs

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

public class Program
{
	private static int i = -1;
	private static List<int> a = new List<int>() {1, 5, 6, 8};
	private static List<int> b = new List<int>() {3, 4, 1, 9};
	private static List<int> result = new List<int>(Enumerable.Repeat(0, a.Count));

	public static void Main(string[] args)
	{
		var threads = Enumerable.Range(0, 3).Select(MakeThread).ToList();
		foreach (var t in threads)
		{
			Console.WriteLine("Starting thread");
			t.Start();
		}
		foreach (var t in threads)
		{
			t.Join();
		}

		foreach (var a in result)
		{
			Console.Write(a + " ");
		}
	}

	private static Thread MakeThread(int z)
	{
		Console.WriteLine("Making thread");
		return new Thread(()=>{				
			while(i < a.Count - 1)
			{
				var idx = Interlocked.Increment(ref i);
				result[idx] = a[idx] + b.Sum();
			}
		});
	}
}