Tukukkk

I've got basic interface, and a class that takes it implementation in constructor, then fetches some data sequentialy or parallely depending on input.

    public interface ISomeService

    {

        List<int> FetchSomeData(int a);

    }      



    public class SomeClass

    {

        private ISomeService sr { get; }



        public SomeClass(ISomeService sr) => this.sr = sr;



        public int DoMagic(bool paralell)

        {

            int sum = 0;

            if (paralell)

            {

                Parallel.For(0, 10, (x) =>

                {

                    sum += sr.FetchSomeData(x).Sum(z => z);

                });

            }

            else

            {

                for (int i = 0; i < 10; i++)

                {

                    sum += sr.FetchSomeData(i).Sum(z => z);

                }

            }

            return sum;

        }

    }

then i've got test:

    [Test]

    public void DoTest()

    {



        var service = new Mock<ISomeService>(MockBehavior.Strict);

        service.Setup(x => x.FetchSomeData(It.IsAny<int>()))

            .Returns(() => new List<int> { 3 });



        var someClass = new SomeClass(service.Object);



        var notParallel = someClass.DoMagic(false);



        var parallel = someClass.DoMagic(true);



        Assert.AreEqual(30, notParallel);



        Assert.AreEqual(30, parallel);

    }

And the problem:

NotParallel results are always the same ( 30 ), but in parallel case there results vary ( sometimes 15, other time 21 etc ).
What may be the problem? How to solve this ?

You are updating the variable sum from multiple threads.
Here are some options how to make this thread-safe: https://social.msdn.microsoft.com/Forums/vstudio/en-US/d87c1085-cacb-4d82-826f-4151bf967f86/parallelfor-with-sum?forum=parallelextensions