C# Academy · Lesson

Parallel.ForEach, PLINQ

Run work in parallel with Parallel.ForEach and build parallel pipelines with PLINQ (AsParallel, WithDegreeOfParallelism, AsOrdered).

Parallel.ForEach, PLINQ is a free C# Academy lesson on CoddyKit — lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why parallel loops

Goal: Use the CPU better.

Parallel.ForEach: action per item
PLINQ: parallel query with operators
Control degree of parallelism
Be careful with side-effects

Parallel.ForEach basics

Parallel.ForEach spreads iterations across threads; each item runs the given action.

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);
    });
  }
}

Control parallelism

Use ParallelOptions.MaxDegreeOfParallelism to limit concurrency when resources are scarce.

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);
    });
  }
}

PLINQ pipeline

PLINQ runs LINQ operators in parallel. ForAll consumes results concurrently (unordered).

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));
  }
}

Ordering controls

Use AsOrdered to preserve order; AsSequential switches back to single-threaded LINQ when needed.

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));
  }
}

Safety & tips

Tips:

Avoid shared mutable state; prefer pure functions.
Use thread-safe collections if you must write.
Do larger batches instead of tiny work items.
Measure: parallelism helps CPU-bound work, not I/O waiting.

Parallel.ForEach vs PLINQ

Quick check: What is a key difference between Parallel.ForEach and PLINQ?

Recap

Recap: Use Parallel.ForEach for per-item actions; use PLINQ to build parallel queries, control concurrency, and optionally preserve order.

Frequently asked questions

Is the “Parallel.ForEach, PLINQ” lesson free?

Yes — the full text of “Parallel.ForEach, PLINQ” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.

What will I learn in “Parallel.ForEach, PLINQ”?

Run work in parallel with Parallel.ForEach and build parallel pipelines with PLINQ (AsParallel, WithDegreeOfParallelism, AsOrdered). You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Parallel.ForEach, PLINQ” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Parallel.ForEach, PLINQ
Producer/consumer with Channels (overview)
Throughput vs latency trade-offs

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