List inheritance behaves strangely

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?

It's how covariance works in .NET for generics. You have to create an intermediate interface. I had the [same question](http://stackoverflow.com/questions/13316838/covariance-issue) a little while ago. — Candide, Dec 10 '12 at 14:33

score 6 · Accepted Answer · answered Dec 10 '12 at 14:33

6

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.

answered Dec 10 '12 at 14:33

Rawling

49,248
7
89
127

`List myList = new List()` would not work because not every A is also a B. `B obj = new A()` wouldn't work either. However, why could a `List` not hold a `C` if `C : A`? I mean, I would not be able to call a `C`-specific method, but at least do anything I could do with an `A`. – MechMK1 Dec 10 '12 at 15:43
1

@David Since you can do anything you could do with an `A`, you can put a `C` in a `List`, but you can't do everything you could do with a `B` so you can't put it in a `List`. – Rawling Dec 10 '12 at 15:47

Warr · Answer 2 · 2012-12-10T14:51:01.650

-2

That simple conversions is not allowed For now use

List<B> lb = new List<B> { ... };
List<A> la = lb.Cast<A>().ToList();

edited Dec 10 '12 at 14:51

answered Dec 10 '12 at 14:32

Warr

118
6

2

"it will be allowed in c# 5.0": No, it is not allowed in C# 5.0. – Daniel Hilgarth Dec 10 '12 at 14:33
`it will be allowed in c# 5.0` No it won't. – Rawling Dec 10 '12 at 14:33
Do you have any good reference on this? C# 5 is in .Net 4.5 and therefore released... – Wilbert Dec 10 '12 at 14:35

List inheritance behaves strangely

2 Answers2