Liskov Substition and Composition

Question

Let say I have a class like this:

public sealed class Foo
{
    public void Bar
    {
       // Do Bar Stuff
    }
}

And I want to extend it to add something beyond what an extension method could do....My only option is composition:

public class SuperFoo
{
    private Foo _internalFoo;

    public SuperFoo()
    {
        _internalFoo = new Foo();        
    }

    public void Bar()
    {
        _internalFoo.Bar();
    }

    public void Baz()
    {
        // Do Baz Stuff
    }
}

While this works, it is a lot of work...however I still run into a problem:

  public void AcceptsAFoo(Foo a)

I can pass in a Foo here, but not a super Foo, because C# has no idea that SuperFoo truly does qualify in the Liskov Substitution sense...This means that my extended class via composition is of very limited use.

So, the only way to fix it is to hope that the original API designers left an interface laying around:

public interface IFoo
{
     public Bar();
}

public sealed class Foo : IFoo
{
     // etc
}

Now, I can implement IFoo on SuperFoo (Which since SuperFoo already implements Foo, is just a matter of changing the signature).

public class SuperFoo : IFoo

And in the perfect world, the methods that consume Foo would consume IFoo's:

public void AcceptsAFoo(IFoo a)

Now, C# understands the relationship between SuperFoo and Foo due to the common interface and all is well.

The big problem is that .NET seals lots of classes that would occasionally be nice to extend, and they don't usually implement a common interface, so API methods that take a Foo would not accept a SuperFoo and you can't add an overload.

So, for all the composition fans out there....How do you get around this limitation?

The only thing I can think of is to expose the internal Foo publicly, so that you can pass it on occasion, but that seems messy.

Answer 1

I found myself asking that same question until I started working on reusable libraries of my own. Many times you wind up with certain classes that just cannot be extended without requiring obscure or arcane sequences of calls from the implementor.

When allowing your class to be extended, you have to ask: if a developer extends my class, and passes this new class to my library, can I transparently work with this new class? Can I work properly with this new class? Is this new class really going to behave the same?

I've found that most of the time the sealed classes in the .Net Framework have certain under-the-hood requirements that you aren't aware of, and that given the current implementation cannot be safely exposed to subclasses.

This doesn't exactly answer your question, but it provides insight as to why not all classes are inheritable in the .Net Framework (and why you should probably entertain sealing some of your classes too).

Answer 2

I'm afraid the short answer is, you can't without doing what is required, i.e. pass the composed instance variable instead.

You could allow an implicit or explicit cast to that type (whose implementation simply passed the composed instance) but this would, IMO be pretty evil.

sixlettervariable's answer is good and I won't rehash it but if you indicated which classes you wished you could extend we might be able to tell you why they prevented it.