Inheritance and Representation

Here's a question I got this morning:

class Alpha<X>
  where X : class
{}
class Bravo<T, U>
  where T : class
  where U : T
{
  Alpha<U> alpha;
}

This gives a compilation error stating that U cannot be used as a type argument for Alpha's type parameter X because U is not known to be a reference type. But surely U is known to be a reference type because U is constrained to be T, and T is constrained to be a reference type. Is the compiler wrong?

Of course not. Bravo<object, int> is perfectly legal and gives a type argument for U which is not a reference type. All the constraint on U says is that U must inherit from T (*). int inherits from object, so it meets the constraint. All struct types inherit from at least two reference types, and some of them inherit from many more. (Enum types inherit from System.Enum, many struct types implement interface types, and so on.)

The right thing for the developer to do here is of course to add the reference type constraint to U as well.

That easily-solved problem got me thinking a bit more deeply about the issue. I think a lot of people don't have a really solid understanding of what "inheritance" means in C#. It is really quite simple: a derived type which inherits from a base type implicitly has all inheritable members of the base type. That's it! If a base type has a member M, then a derived type has a member M as well. (**)

People sometimes ask me if private members are inherited; surely not! What would that even mean? But yes, private members are inherited, though most of the time it makes no difference because the private member cannot be accessed outside of its accessibility domain. However, if the derived class is inside the accessibility domain then it becomes clear that yes, private members are inherited:

class B
{
  private int x;
  private class D : B
  {

D inherits x from B, and since D is inside the accessibility domain of x, it can use x no problem.

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