ConcurrentBatchAdder.cs

internal class ConcurrentBatchAdder : IIntegerAdder
{

    private const int degreeOfParallelism = 4;

    public int Add(int[] values)
    {
        var countPerSegment = values.Length / degreeOfParallelism + 1;
        int[,] segments = SplitIntoSegments(values, countPerSegment);
        var sums = new int[degreeOfParallelism];
        Action<object> addSegment = segmentIndexParam =>
        {
            int segmentIndexParamAsInt = (int)segmentIndexParam;
            for (var i = 0; i < countPerSegment; i++)
            {
                sums[segmentIndexParamAsInt] += segments[segmentIndexParamAsInt, i];
            }
        };
        var tasks = new Task[degreeOfParallelism];
        for (var i = 0; i < degreeOfParallelism; i++)
        {
            tasks[i] = Task.Factory.StartNew(addSegment, i);
        }
        Task.WaitAll(tasks);
        int sum = 0;
        for (var i = 0; i < sums.Length; i++)
        {
            sum += sums[i];
        }
        return sum;
    }

    private static int[,] SplitIntoSegments(int[] values, int countPerSegment)
    {
        //if values.Length isn't divisible by the degree of parallelism, the "extra" elements in the rectangular array will be 0
        var segments = new int[degreeOfParallelism, countPerSegment]; 
        var segmentIndex = 0;
        var itemIndex = 0;
        foreach (var value in values)
        {
            segments[segmentIndex, itemIndex] = value;
            if (itemIndex == countPerSegment - 1)
            {
                segmentIndex += 1;
                itemIndex = 0;
            }
            else
            {
                itemIndex += 1;
            }
        }

        return segments;
    }
}