Check if an IEnumerable has less than a certain number of items without causing any unnecessary evaluation?

Question

Sometimes I expect a certain range of items and need to do some validation to ensure that I am within that range. The most obvious way to do this is to just compare the number of items in the collection with the range.

public static bool IsWithinRange<T>(this IEnumerable<T> enumerable, int max)
{
    return enumerable.Count() <= max;
}

Although, my understanding is that the linq Count() method will evaluate the entire enumerable before returning a result. Ideally I would only cause evaluation on the minimal number of items to get my result.

What would be the best way to ensure that an enumerable has less than a certain number of items without causing any unnecessary evaluation?

Answer 1

Don't use Count(), as you already know, the entire collection would have to be traversed in general.

You can do this instead:

public static bool IsWithinRange<T>(this IEnumerable<T> enumerable, int max)
{
    return !enumerable.Skip(max).Any();
}

Note you will still have to enumerate over the first max items in the collection, that's unavoidable unless you try to make some assumptions about the underlying collection.

---

To really optimize this further, you can check if the underlying type is an ICollection<> or ICollection in order to access the Count property. That way you don't have to enumerate over the items at all. Otherwise fallback on enumerating the items.

public static bool IsWithinRange<T>(this IEnumerable<T> enumerable, int max)
{
    var asCollection = enumerable as System.Collections.ICollection;
    if (asCollection != null) return asCollection.Count <= max;
    var asGenericCollection = enumerable as ICollection<T>;
    if (asGenericCollection != null) return asGenericCollection.Count <= max;
    return !enumerable.Skip(max).Any();
}

Of course this isn't exactly free as you're doing additional checks, but it beats having to enumerate over the collection if at all possible, particularly if max is large.