Parallel.ForEach, PLINQ

using System;
using System.Threading.Tasks;

public class Program
{
  public static void Main(string[] args)
  {
    int[] data = new int[] { 1, 2, 3, 4, 5, 6 };

    // Runs iterations concurrently on thread pool
    Parallel.ForEach(data, x =>
    {
      int y = x * x; // small CPU work
      Console.WriteLine("x=" + x + " y=" + y + " on " + Environment.CurrentManagedThreadId);
    });
  }
}

using System;
using System.Threading;
using System.Threading.Tasks;

public class Program
{
  public static void Main(string[] args)
  {
    int[] data = new int[] { 1, 2, 3, 4, 5, 6 };
    ParallelOptions opt = new ParallelOptions();
    opt.MaxDegreeOfParallelism = 2; // limit concurrency

    Parallel.ForEach(data, opt, x =>
    {
      Console.WriteLine("Start " + x);
      Thread.Sleep(150); // simulate work
      Console.WriteLine("Done " + x);
    });
  }
}

using System;
using System.Linq;

public class Program
{
  public static void Main(string[] args)
  {
    int[] nums = Enumerable.Range(1, 20).ToArray();

    // Filter then project in parallel
    var query =
      nums
        .AsParallel()
        .WithDegreeOfParallelism(2)
        .Where(n => n % 2 == 0)
        .Select(n => n * n);

    // ForAll consumes in parallel; order not guaranteed
    query.ForAll(sq => Console.WriteLine(sq));
  }
}

using System;
using System.Linq;

public class Program
{
  public static void Main(string[] args)
  {
    int[] nums = new int[] { 5, 4, 3, 2, 1 };

    // Preserve original sequence order for the output
    var ordered =
      nums.AsParallel().AsOrdered().Select(n => n * 10).ToArray();

    // Switch back to sequential for a section if needed
    var seq =
      nums.AsParallel().Select(n => n + 1).AsSequential().Where(n => n > 3).ToArray();

    Console.WriteLine("Ordered: " + string.Join(",", ordered));
    Console.WriteLine("Sequential tail: " + string.Join(",", seq));
  }
}