SelectParallelPreserveOrder

//This achieves 3 things:
//1. Processes in parallel
//2. Preserves original order
//3. Returns items immediately after they are available
public static IEnumerable<TOutput> SelectParallelPreserveOrder<TInput,TOutput>(
    this IEnumerable<TInput> source,
    Func<TInput, TOutput> selector,
    int? degree = null)
{
    var dop = Math.Max(degree ?? Environment.ProcessorCount, 1);

    return source
        .AsParallel()
        .AsOrdered()                                // preserve source order
        .WithDegreeOfParallelism(dop)
        .WithMergeOptions(ParallelMergeOptions.NotBuffered) // stream ASAP
        .Select(selector)
        .AsSequential();                            // return as IEnumerable<TOutput>
}