Question

I’ve got two extensions for IEnumerable:

public static class IEnumerableGenericExtensions
{
    public static IEnumerable<IEnumerable<T>> InSetsOf<T>(this IEnumerable<T> source, int max)
    {
        List<T> toReturn = new List<T>(max);
        foreach (var item in source)
        {
            toReturn.Add(item);
            if (toReturn.Count == max)
            {
                yield return toReturn;
                toReturn = new List<T>(max);
            }
        }
        if (toReturn.Any())
        {
            yield return toReturn;
        }
    }

    public static int IndexOf<T>(this IEnumerable<T> source, Predicate<T> searchPredicate)
    {
        int i = 0;
        foreach (var item in source)
            if (searchPredicate(item))
                return i;
            else
                i++;

        return -1;
    }
}

Then I write this code:

        Pages = history.InSetsOf<Message>(500);
        var index = Pages.IndexOf(x => x == Pages.ElementAt(0));

where
public class History : IEnumerable
But as a result I’ve got not ‘0’ as I’ve expected, but ‘-1’. I cant understand – why so?

Answer 1

When you write Pages.IndexOf(x => x == Pages.ElementAt(0));, you actually run InSetsOf many times, due to deferred execution (aka lazy). To expand:

• Pages = history.InSetsOf<Message>(500) – this line doesn’t run InSetsOf at all.
• Pages.IndexOf – Iterates over Pages, so it starts executing InSetsOf once.
• x == Pages.ElementAt(0) – this executes InSetsOf again, once for every element in the collection of Pages (or at least until searchPredicate return true, which doesn’t happen here).

Each time you run InSetsOf you create a new list (specifically, a new first list, because you use ElementAt(0)). These are two different objects, so comparison of == between them fails.

An extremely simple fix would be to return a list, so Pages is not a deferred query, but a concrete collection:

Pages = history.InSetsOf<Message>(500).ToList();

Another option is to use SequenceEqual, though I’d recommend caching the first element anyway:

Pages = history.InSetsOf<Message>(500);
var firstPage = Pages.FirstOrDefault();
var index = Pages.IndexOf(x => x.SequenceEqual(firstPage));