C# - For vs Foreach - Huge performance difference

Question

i was making some optimizations to an algorithm that finds the smallest number that is bigger than X, in a given array, but then a i stumbled on a strange difference. On the

Answer 1

IEnumerable is not indexable.

The Count() and ElementAt() extension methods that you call in every iteration of your for loop are O(n); they need to loop through the collection to find the count or the nth element.

Moral: Know thy collection types.