Answer by Paulustrious for LINQ equivalent of foreach for IEnumerable

Now we have the option of...

        ParallelOptions parallelOptions = new ParallelOptions();
        parallelOptions.MaxDegreeOfParallelism = 4;
#if DEBUG
        parallelOptions.MaxDegreeOfParallelism = 1;
#endif
        Parallel.ForEach(bookIdList, parallelOptions, bookID => UpdateStockCount(bookID));

Of course, this opens up a whole new can of threadworms.

ps (Sorry about the fonts, it's what the system decided)