Q&A: .NET Cross Language Inheritance?

August 23rd, 2001

Q: D.J. Stachniak writes:

I missed the Conference.NET, but I've been reading through the PowerPoint presentations posted on DevelopMentor's web site. I noticed in the C++ In A Managed World presentation that there is a slide (page 62 specifically) which says ".NET supports real inheritance across languages - Although, that doesn't mean you should use it..."

Would anyone like to take a crack and explain to me why cross language inheritance might be bad?

A: Chris responds:

At the Microsoft PDC in 1993 (also known as the "COM PDC"), Kraig B. introduced COM by saying that it didn't have inheritance, but that was a good thing, because of the fragile base class problem. This problem says that derived objects, by definition, need to have incestuous knowledge of their base classes, including full understanding of the implied contract (which often requires to source to determine). What made this relationship fragile was that reliance on the implied contract caused all kinds of trouble when it changed, implying that it was knowledge of the source that caused this problem.

Of course, that's not the case. We often experiment with components that we don't have the source for and made decisions based on implied contracts that can easily change version to version. I would call this the "fragile use" problem, but it's more commonly called "DLL Hell." The idea that components can be replaced in the field without breaking things has long been known as a fallacy, and it doesn't matter if you've got the source or whether you're doing inheritance, aggregation or mere creation.

Eiffel's Design by Contract is a good step towards detecting problems, but that's an Eiffel-only thing. A good step towards reducing DLL Hell is the aggressive versioning and side-by-side installation of multiple versions that .NET supports. If your client or derived class binds to a specific version of a component, then there's no need to worry about it changing out from under you. Lack of source makes it harder to derive than to merely use, but assuming you can make things work empirically, cross-language inheritance might actually work in .NET (but don't hold me to that : ).