A parallelism issue

Question

I've written 3 different ways of computing a sum of an array of integer, however, I'm getting a different result for the third method.

Initialization:

        int n = 100;
        int[] mArray = new int[n];
        for (int i = 0; i < mArray.Length; i++)          
            mArray[i] = 1;

First:

        int sum1 = mArray.Sum();
        Console.WriteLine("sum1 " + sum1);

Second:

        int sum2 = 0;
        for (int i = 0; i < mArray.Length; i++)
            sum2 += mArray[i];
        Console.WriteLine("sum2 " + sum2);

Third:

        int sum3 = 0;
        Parallel.ForEach(mArray, item =>
        {
            sum3 += item;
        });
        Console.WriteLine("sum3 " + sum3);

Obviously, the 3 approaches gave the same output as seen below :

However, when n is increased (e.g., n = 30000), the third approach gives surprisingly false results

NB: I've tested the approaches using ConcurrentBag which is a thread-safe collection. I suppose, there is no overflow issue. The code is tested on a windows 10 x64 computer (Intel core I-7 @ 3.30ghz)

It would be interesting to understand why Parallel.For behaves differently.

Answer 1

I'have Nick's solution and it fixed the problem, however, there was a performance issue when using lock (objLock) { sum3 += item; } directly in the Parallel.ForEach, as shown in the figure below

Fortunatly, using Parallel Aggregation Operations, as properly defined in .Net solved the issue. Here is the code

        object locker = new object();
        double sum4= 0;
        Parallel.ForEach(mArray,
                        () => 0.0,                 // Initialize the local value.
                        (i, state, localResult) => localResult + i, localTotal =>   // Body delegate which returns the new local total.                                                                                                                                           // Add the local value
                            {
                                lock (locker) sum4+= localTotal;
                            }    // to the master value.
                        );