Is casting an IEnumerable to an ArrayList O(N) or O(1)?

Question

When I do this cast:

private IEnumerable objects;
ArrayList castedObjects = (ArrayList)objects;

Is this a "direct" cast, or the enumerable is converted into an ArrayList via an internal method (that presumably loops through all the elements)?

Answer 1

This is either a "direct" cast (if the underlying object is actually an instance of ArrayList), or you'll get an exception. No implicit creating of an ArrayList object is done "behind the scene".

Answer 2

This is a direct cast. It will only work if the object is an ArrayList already, for example:

IEnumerable objects = new ArrayList();
ArrayList castedObjects = (ArrayList)objects;

If the object is not an ArrayList but some other object that implements IEnumerable, the cast will fail.

You can create an ArrayList from an ICollection, for example:

ICollection objects = new string[] { "a", "b" };
ArrayList castedObjects = new ArrayList(objects);

This will loop through the collection and copy the items to the ArrayList, so it's an O(n) operation.

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Generally you should not use the ArrayList class at all, but the generic List<T> class that offers strict typing.