C# Online Compiler | .NET Fiddle

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


					
public class Program
{
	public static void Main()
	{
		//SimpleParallelFor();
		ParallelForWithThreadLocalVariables();
	}
	
	private static void SimpleParallelFor() 
	{
		ParallelLoopResult result = Parallel.For(0, 100, index => CalculateSum(index));
		Console.WriteLine(result.IsCompleted);
	}
	
	private static void CalculateSum(int index) 
	{
		Random rnd = new Random(index * 100000);
		Byte[] bytes = new Byte[100];
		rnd.NextBytes(bytes);
		int sum = 0;
		foreach(var byt in bytes)
		{
			sum += byt;
		}
		Console.WriteLine($"Index: {index}, Sum: {sum}");
	}
	
	private static void ParallelForWithThreadLocalVariables() 
	{
		int[] nums = Enumerable.Range(0, 1000000).ToArray();
        long total = 0;

        Parallel.For<long>(
			0, 
			nums.Length, 
			() => 0, 
			(index, loopState, subtotal) =>
        	{
            	subtotal += nums[index];
            	return subtotal;
        	},
            (subTotal) => Interlocked.Add(ref total, subTotal)
        );

        Console.WriteLine("The total is {0:N0}", total);
	}
}

 
using System;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
​
​
                    
public class Program
{
    public static void Main()
    {
        //SimpleParallelFor();
        ParallelForWithThreadLocalVariables();
    }
    
    private static void SimpleParallelFor() 
    {
        ParallelLoopResult result = Parallel.For(0, 100, index => CalculateSum(index));
        Console.WriteLine(result.IsCompleted);
    }
    

Cached Result

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