ParallelAdder.cs

internal class ParallelAdder : IIntegerAdder
{
    public int Add(int[] values)
    {
        var queue = new ConcurrentQueue<int>(values);
        int overallSum = 0;
        Action addFromQueue = () =>
        {
            int localSum = 0;
            while (queue.TryDequeue(out int localValue))
            {
                localSum += localValue;
            }
            Interlocked.Add(ref overallSum, localSum);
        };
        Parallel.Invoke(addFromQueue, addFromQueue, addFromQueue, addFromQueue);
        return overallSum;
    }