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How to: Write a Simple Parallel.ForEach Loop

This example shows how to use a Parallel.ForEach loop to enable data parallelism over any System.Collections.IEnumerable or System.Collections.Generic.IEnumerable<T> data source.


This documentation uses lambda expressions to define delegates in PLINQ. If you are not familiar with lambda expressions in C# or Visual Basic, see Lambda Expressions in PLINQ and TPL.

namespace ForEachDemo
    using System;
    using System.Drawing; // requires system.Drawing.dll
    using System.IO;
    using System.Threading;
    using System.Threading.Tasks;

    class SimpleForEach
        static void Main()
            // A simple source for demonstration purposes. Modify this path as necessary.
            string[] files = System.IO.Directory.GetFiles(@"C:\Users\Public\Pictures\Sample Pictures", "*.jpg");
            string newDir = @"C:\Users\Public\Pictures\Sample Pictures\Modified";

            //  Method signature: Parallel.ForEach(IEnumerable<TSource> source, Action<TSource> body)
            Parallel.ForEach(files, currentFile =>
                // The more computational work you do here, the greater 
                // the speedup compared to a sequential foreach loop.
                string filename = System.IO.Path.GetFileName(currentFile);
                System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(currentFile);

                bitmap.Save(System.IO.Path.Combine(newDir, filename));

                // Peek behind the scenes to see how work is parallelized.
                // But be aware: Thread contention for the Console slows down parallel loops!!!
                Console.WriteLine("Processing {0} on thread {1}", filename,

            } //close lambda expression
                 ); //close method invocation

            // Keep the console window open in debug mode.
            Console.WriteLine("Processing complete. Press any key to exit.");

A ForEach loop works like a For loop. The source collection is partitioned and the work is scheduled on multiple threads based on the system environment. The more processors on the system, the faster the parallel method runs. For some source collections, a sequential loop may be faster, depending on the size of the source, and the kind of work being performed. For more information about performance, see Potential Pitfalls in Data and Task Parallelism

For more information about parallel loops, see How to: Write a Simple Parallel.For Loop.

To use ForEach with a non-generic collection, you can use the Cast<TResult> extension method to convert the collection to a generic collection, as shown in the following example:

    currentElement =>

You can also use Parallel LINQ (PLINQ) to parallelize processing of IEnumerable<T> data sources. PLINQ enables you to use declarative query syntax to express the loop behavior. For more information, see Parallel LINQ (PLINQ).

  • Copy and paste this code into a Visual Studio 2010 Console Application project.

  • Add a reference to System.Drawing.dll

  • Press F5