Question

Does the .Last() extension method take into account if it’s called on an IList? I’m just wondering if there’s a significant performance difference between these:

IList<int> numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

int lastNumber1 = numbers.Last();
int lastNumber2 = numbers[numbers.Count-1];

Intuition tells me that the first alternative is O(n) but the second is O(1). Is .Last() “smart” enough to try casting it to an IList?

Answer 1

Probably not, as it can do list[list.count-1]

Verified by reflector:

public static TSource Last<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    IList<TSource> list = source as IList<TSource>;
    if (list != null)
    {
        int count = list.Count;
        if (count > 0)
        {
            return list[count - 1];
        }
    }
    ...
}