polymorphic resolution of generic parameters in Unity registerType

Question

I am trying to do the following and failing:

class base {}

class derived1 : base {}

class derived2 : base {}

interface itr<t>
where t : class, base
{}

class c1: itr<derived1>
{}

class c2 : itr<derived2>
{}  

//The following 2 registrations fail:

_unityContainer.RegisterType<itr<base>, c1>("c1");

_unityContainer.RegisterType<itr<base>, c2>("c2");

The error I get is that the second parameter in the above registrations cannot be typecast into the first parameter and this registration is not valid. Any suggestions on how I can do this?
I need to do the above instead of registering with the derived1 or derived2 classes as generic parameters because while resolving I don't want to have to know the exact derived type I am resolving. I only want to work with base type methods polymorphically.

Answer 1

You cannot do this because generics are not covariant. That is, the type itr<derived1> cannot be converted to itr<base>, even though derived1 can be converted to base.

Here's an example of why, using the framework List<T> class:

List<string> list1 = new List<string>();
list1.Add("Hello, World!");

// This cast will fail, because the following line would then be legal:
List<object> list2 = (List<object>)list1; 

// ints are objects, but they are not strings!
list2.Add(1);

So accessing list1[1] would return a boxed int in a list that was declared to contain strings.

Therefore, this cast is not allowed since it would break the type system.

(As a side note, in the clause where t : class, base, you do not need to specify class. base itself is a reference type, so the class constraint is redundant.)