Question

Possible Duplicate:
C# variance problem: Assigning List<Derived> as List<Base>

I have a problem with list inheritance. It looks like a generic List with more specified members can’t converted to a list with its base members.
Look at this:

class A
{
    public int aValue {get; set;}
}

class B : A
{
    public int anotherValue {get; set;}
}

You might now expect that a List<B> is also a List<A> but that’s not the case.
List<A> myList = new List<B>() is not possible. Not even List<A> myList = (List<A>)new List<B>() Am I missing some basic concept here after 3 years in object oriented programming?

Answer 1

Yup!

Assume you could do

List<A> myList = new List<B>();

Then assume you have a class

class C : A { public int aDifferentValue { get; set; } }

A C is an A, so you would expect to be able to call myList.Add(new C()) as myList thinks it is a List<A>.

But a C is not a B so a myList – which is really a List<B> – can’t hold a C.

---

Conversely, suppose you could do

List<B> myList = new List<A>();

You can happily call myList.Add(new B()) because a B is an A.

But suppose something else stuck a C in your list (as C is an A).

Then myList[0] might return a C – which is not a B.