Why, when and how need to use Interlocked.Add? By Example

Try the code below.

 

class Test

  {

    public static void Add(ref long total, long finalResult) { total = finalResult; }

 

    static void Main()

    {

      int[] nums = Enumerable.Range(0, 1000000).ToArray();

      long total = 0;

 

      // First type parameter is the type of the source elements

      // Second type parameter is the type of the local data (subtotal)

      Parallel.ForEach<int, long>(nums, // source collection

                                  () => 0, // method to initialize the local variable

                                  (j, loop, subtotal) => // method invoked by the loop on each iteration

                                  {

                                    subtotal += nums[j]; //modify local variable

                                    return subtotal; // value to be passed to next iteration

                                  },

        // Method to be executed when all loops have completed.

        // finalResult is the final value of subtotal. supplied by the ForEach method.

                                    (finalResult) => Add(ref total, finalResult)

                                  );

 

      Console.WriteLine("The total from Parallel.ForEach is {0}", total);

      Console.WriteLine("Press any key to exit");

      Console.ReadKey();

    }

 

Run multiple times. What happen to the result?

  class Test

  {

    static void Main()

    {

      int[] nums = Enumerable.Range(0, 1000000).ToArray();

      long total = 0;

 

      // First type parameter is the type of the source elements

      // Second type parameter is the type of the local data (subtotal)

      Parallel.ForEach<int, long>(nums, // source collection

                                  () => 0, // method to initialize the local variable

                                  (j, loop, subtotal) => // method invoked by the loop on each iteration

                                  {

                                    subtotal += nums[j]; //modify local variable

                                    return subtotal; // value to be passed to next iteration

                                  },

        // Method to be executed when all loops have completed.

        // finalResult is the final value of subtotal. supplied by the ForEach method.

                                  (finalResult) => Interlocked.Add(ref total, finalResult)

                                  );

 

      Console.WriteLine("The total from Parallel.ForEach is {0}", total);

      Console.WriteLine("Press any key to exit");

      Console.ReadKey();

    }

  }

 

It is obvious that you cannot get the consistent result because of parallelism is not linear. Hence the Interlocked.Add is needed to get the result all the time.

About chanmingman

Since March 2011 Microsoft Live Spaces migrated to Wordpress (http://www.pcworld.com/article/206455/Microsoft_Live_Spaces_Moves_to_WordPress_An_FAQ.html) till now, I have is over 1 million viewers. This blog is about more than 50% telling you how to resolve error messages, especial for Microsoft products. The blog also has a lot of guidance teaching you how to get stated certain Microsoft technologies. The blog also uses as a help to keep my memory. The blog is never meant to give people consulting services or silver bullet solutions. It is a contribution to the community. Thanks for your support over the years. Ming Man is Microsoft MVP since year 2006. He is a software development manager for a multinational company. With 25 years of experience in the IT field, he has developed system using Clipper, COBOL, VB5, VB6, VB.NET, Java and C #. He has been using Visual Studio (.NET) since the Beta back in year 2000. He and the team have developed many projects using .NET platform such as SCM, and HR based applications. He is familiar with the N-Tier design of business application and is also an expert with database experience in MS SQL, Oracle and AS 400.
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