Question

I know that String extension methods return a String and do not actually affect the variable calling the extension method (so it’s immutable) – but how do I tell whether other extension methods do or not? For example, I’m working with a List<NewsItem> – and I need to order this list by date descending, so I’ve written this code:

newsItems.OrderByDescending(o => o.Date);

Does this affect the newsItems list or just return the IOrderedEnumerable??

In other words, should the above code actually read:

newsItems = newsItems.OrderByDescending(o => o.Date).ToList();

??

Thanks,

Dan

Answer 1

OrderByDescending is an extension method on IEnumerable<T>, which itself provides read-only access. Of course, the extension method could cast to List<T>, but basically none of the LINQ to Objects extension methods affect their target (assuming the targets aren’t affected by iteration, of course).

LINQ is designed in a functional style – methods return a new view on the data rather than changing the target they’re called on.

EDIT: As noted by drew, the [Pure] attribute can be used as an indication of this, and some tools will pick up on it. However:

• You still need to trust that the [Pure] attribute has been correctly applied
• It’s not always easy to tell whether or not there is a [Pure] attribute (it doesn’t show up in the docs I linked to, for example)
• You should probably be reading the documentation for anything you call anyway, if you aren’t confident in what it does