Speed up loop using multithreading in C# (Question)

foreach (String item in ListOfStrings)
{
    result.add(CalculateSmth(item));
}

Parallel.Foreach (ListOfStrings, (item) =>
    result.add(CalculateSmth(item));
);

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

namespace noocyte.Threading
{
    class CalcState
    {
        public CalcState(ManualResetEvent reset, string input) {
            Reset = reset;
            Input = input;
        }
        public ManualResetEvent Reset { get; private set; }
        public string Input { get; set; }
    }

    class CalculateMT
    {
        List<string> result = new List<string>();
        List<ManualResetEvent> events = new List<ManualResetEvent>();

        private void Calc() {
            List<string> aList = new List<string>();
            aList.Add("test");

            foreach (var item in aList)
            {
                CalcState cs = new CalcState(new ManualResetEvent(false), item);
                events.Add(cs.Reset);
                ThreadPool.QueueUserWorkItem(new WaitCallback(Calculate), cs);
            }
            WaitHandle.WaitAll(events.ToArray());
        }

        private void Calculate(object s)
        {
            CalcState cs = s as CalcState;
            cs.Reset.Set();
            result.Add(cs.Input);
        }
    }
}

List<string> work = (some list with lots of strings)

// Split the work in two
List<string> odd = new List<string>();
List<string> even = new List<string>();
for (int i = 0; i < work.Count; i++)
{
    if (i % 2 == 0)
    {
        even.Add(work[i]);
    }
    else
    {
        odd.Add(work[i]);
    }
}

// Set up to worker delegates
List<Foo> oddResult = new List<Foo>();
Action oddWork = delegate { foreach (string item in odd) oddResult.Add(CalculateSmth(item)); };

List<Foo> evenResult = new List<Foo>();
Action evenWork = delegate { foreach (string item in even) evenResult.Add(CalculateSmth(item)); };

// Run two delegates asynchronously
IAsyncResult evenHandle = evenWork.BeginInvoke(null, null);
IAsyncResult oddHandle = oddWork.BeginInvoke(null, null);

// Wait for both to finish
evenWork.EndInvoke(evenHandle);
oddWork.EndInvoke(oddHandle);

// Merge the results from the two jobs
List<Foo> allResults = new List<Foo>();
allResults.AddRange(oddResult);
allResults.AddRange(evenResult);

return allResults;