I have the following extension method:

    public static IEnumerable<T> Apply<T>(
        [NotNull] this IEnumerable<T> source,
        [NotNull] Action<T> action)
        where T : class
    {
        source.CheckArgumentNull("source");
        action.CheckArgumentNull("action");
        return source.ApplyIterator(action);
    }

    private static IEnumerable<T> ApplyIterator<T>(this IEnumerable<T> source, Action<T> action)
        where T : class
    {
        foreach (var item in source)
        {
            action(item);
            yield return item;
        }
    }

It just applies an action to each item of the sequence before returning it.

I was wondering if I should apply the Pure attribute (from Resharper annotations) to this method, and I can see arguments for and against it.

Pros:

• strictly speaking, it is pure; just calling it on a sequence doesn't alter the sequence (it returns a new sequence) or make any observable state change
• calling it without using the result is clearly a mistake, since it has no effect unless the sequence is enumerated, so I'd like Resharper to warn me if I do that.

Cons:

• even though the Apply method itself is pure, enumerating the resulting sequence will make observable state changes (which is the point of the method). For instance, items.Apply(i => i.Count++) will change the values of the items every time it's enumerated. So applying the Pure attribute is probably misleading...

What do you think? Should I apply the attribute or not?