CsharpHow-ToBeginner · 3 min read

How to Use Parallel.For in C# for Simple Parallel Loops

Use Parallel.For in C# to run a loop where iterations execute in parallel on multiple threads. It takes a start index, an end index, and a loop body action, making it easy to speed up independent tasks without manual thread management.

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Syntax

The Parallel.For method runs a loop in parallel. It requires three main parts:

startInclusive: The first index to start looping from.
endExclusive: The index to stop before (not included).
body: A delegate (action) that defines what to do for each index.

This method automatically manages threads to run iterations concurrently.

csharp

Parallel.For(int startInclusive, int endExclusive, Action<int> body);

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Example

This example shows how to use Parallel.For to print numbers from 0 to 9 in parallel. The order may vary because iterations run on different threads.

csharp

using System;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        Parallel.For(0, 10, i =>
        {
            Console.WriteLine($"Processing number {i} on thread {System.Threading.Thread.CurrentThread.ManagedThreadId}");
        });
    }
}

Output

Processing number 0 on thread 4 Processing number 1 on thread 5 Processing number 2 on thread 6 Processing number 3 on thread 4 Processing number 4 on thread 5 Processing number 5 on thread 6 Processing number 6 on thread 4 Processing number 7 on thread 5 Processing number 8 on thread 6 Processing number 9 on thread 4

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Common Pitfalls

Common mistakes when using Parallel.For include:

Modifying shared variables without synchronization, causing data races.
Assuming the loop runs in order; iterations run concurrently and order is not guaranteed.
Using Parallel.For for very small or quick tasks, which can add overhead and slow down performance.

Always ensure thread safety when accessing shared data.

csharp

using System;
using System.Threading.Tasks;

class Program
{
    static int total = 0;

    static void Main()
    {
        // Wrong: modifying shared variable without lock
        Parallel.For(0, 1000, i =>
        {
            total += i; // This can cause wrong results
        });
        Console.WriteLine($"Wrong total: {total}");

        // Right: use lock to protect shared variable
        total = 0;
        object lockObj = new object();
        Parallel.For(0, 1000, i =>
        {
            lock (lockObj)
            {
                total += i;
            }
        });
        Console.WriteLine($"Correct total: {total}");
    }
}

Output

Wrong total: 123456 Correct total: 499500

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Quick Reference

Parallel.For Cheat Sheet:

startInclusive: loop start index (inclusive)
endExclusive: loop end index (exclusive)
body: action to run for each index
Runs iterations in parallel on multiple threads
Order of execution is not guaranteed
Use synchronization for shared data

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Key Takeaways

Parallel.For runs loop iterations concurrently to speed up independent tasks.

Always protect shared data with locks or thread-safe methods inside Parallel.For.

The loop order is not guaranteed; do not rely on sequential execution.

Use Parallel.For for CPU-bound tasks that benefit from parallelism.

Avoid Parallel.For overhead for very small or quick loops.