C# variance problem: Assigning List as List<Base>

Question

Look at the following example (partially taken from MSDN Blog):

class Animal { }
class Giraffe : Animal { }

static void Main(string[] args)
{
    // Array a         


        

Answer 1

GenericClass and GenericClass and two distinct closed constructed generic types of GenericClass<> open generic type and they not inherit one from the other.

So you can't cast GenericClass to GenericClass even if B inherits from A.

It is like you ask to cast this:

class A2 : A1;
class B2 : B1;

var a2 = new A2();
var b2 = new B2();
var x = (A1)b2;

GenericClass and GenericClass are as distinct as A1 and B2.

But they are all object.

And since there is no diamond operator in C# yet, you can't use true polymorphism on open generic type underlying to closed constructed types like this:

var x = (GenericClass<>)b;

You can't create list like this:

List<> list;

You can't do polymorphism on such list... and it is a lack in genericity here.

For example, in C# you can't create a List> instance to have some Washer and some Washer to operate Wash() on them... and to do that you need an ugly interface pattern...

Generics -Open and closed constructed Types

About the lack of true generic polymorphism and the missing diamond operator in C#