C# foreach behavior with derived classes?

Question

Right now I have a relatively simple class setup:

class A{
//stuff
}
class B:A{
//more stuff
}
public List<A> ListOfObjects;

What would happen if I do

foreach(B i in ListOfObjects)

would I get only objects of type B? Would it apply some OO magic and convert all As to Bs? Would this even work?

Answer 1

If the list had A's in it (or other things that aren't B or subclasses of B), then it would simply break with an invalid-cast. You probably want:

foreach(B i in ListOfObjects.OfType<B>()) {...}

in .NET 3.5. (I'm assuming that the list itself will be non-null, btw)

Answer 2

I personally use var as the loop variable in foreach under all circumstances, to avoid any possibility of an invalid runtime cast. That way, the type of the loop variable will be the static type of the collection's items; if you want something else, use OfType to perform a safe filtering cast.

Some more explanation here.