C# Online Compiler | .NET Fiddle

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Linq;
using System.Diagnostics;
using System.Threading.Tasks;
					
public class Program
{
	public static void Main()
	{
		var limit = 2000000;
		var numbers = Enumerable.Range(0, limit).ToList();
		
		var watch = Stopwatch.StartNew();
		var primeNumbers = GetPrimeList(numbers);
		watch.Stop();
		
		var watch2 = Stopwatch.StartNew();
		var primeNumbersFromParallel = GetPrimeListWithParallel(numbers);
		
		
		Console.WriteLine($"Using Normal ForLoop, Total Time: {watch.ElapsedMilliseconds}, Total Prime: {primeNumbers.Count}");
		Console.WriteLine($"Using Parallel ForEachLoop, Total Time: {watch2.ElapsedMilliseconds}, Total Prime: {primeNumbersFromParallel.Count}");
	}
	
	private static IList<int> GetPrimeList(IList<int> numbers) => numbers.Where(IsPrime).ToList();
	
	private static IList<int> GetPrimeListWithParallel(IList<int> numbers) 
	{
			var primeNumbers = new ConcurrentBag<int>();
			Parallel.ForEach(numbers, number => {
				if(IsPrime(number))
					primeNumbers.Add(number);
			});
			return primeNumbers.ToList();
	}
	
	private static bool IsPrime(int number) 
	{
		if(number < 2)
			return false;
		
		for(var divisor = 2; divisor <= Math.Sqrt(number); divisor++) 
		{
			if(number % divisor == 0)
				return false;
		}
		return true;
	}
}

 
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Linq;
using System.Diagnostics;
using System.Threading.Tasks;
                    
public class Program
{
    public static void Main()
    {
        var limit = 2000000;
        var numbers = Enumerable.Range(0, limit).ToList();
        
        var watch = Stopwatch.StartNew();
        var primeNumbers = GetPrimeList(numbers);
        watch.Stop();
        
        var watch2 = Stopwatch.StartNew();
        var primeNumbersFromParallel = GetPrimeListWithParallel(numbers);
        
        
        Console.WriteLine($"Using Normal ForLoop, Total Time: {watch.ElapsedMilliseconds}, Total Prime: {primeNumbers.Count}");

Cached Result

directory not found exception handled
Tasks Exception: ArgumentNullException Value cannot be null. (Parameter 'path')