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Virtual Has Two Meanings

Anders Hejlsberg: When you say "virtual," you can mean one of two things. If you did not inherit a method of the same signature, then this is a new virtual method. That's one meaning. Otherwise it is an override of an inherited method. That's the other meaning.

From a versioning perspective, it is important that the programmer indicate their intent when they declare a method virtual. In C#, for example, you must explicitly indicate which meaning of virtual you intend. To declare a new virtual method, you just mark it virtual. But to override an existing virtual method, you must say override.

As a result, C# doesn't have the particular versioning problem I described earlier in which we introduce a method in a base class that you already have in a derived class. In your class, you would have declared foo virtual. Now we introduce a new virtual foo. Well, that's fine. Now there are two virtual foos. There are two VTBL slots. The derived foo hides the base foo, but that's fine. The base foo wasn't even there when the derived foo was written, so it's not like there's anything wrong with hiding this new functionality. And things continue to work the way they're supposed to.

Bruce Eckel: So you saw this versioning problem happen often enough in practice, that you decided to address it? I remember you did a similar thing in Delphi.

Anders Hejlsberg: Yes.

Bruce Eckel: You're perspective on languages is quite different from the other people that I've talked to. It's very pragmatic.

Anders Hejlsberg: I have always described myself as a pragmatic guy. It's funny, because versioning ended up being one of the pillars of our language design. It shows up in how you override virtual methods in C#. Also, the way we do overload resolution in C# is different from any other language I know of, for reasons of versioning. Whenever we looked at designing a particular feature, we would always cross check with versioning. We would ask, "How does versioning change this? How does this function from a versioning perspective?" It turns out that most language design before has given very little thought to that.

Bruce Eckel: Were you concerned about versioning primarily because of the DLL hell problem?

Anders Hejlsberg: Yes, but also because of changes I've observed over the years. Ten to fifteen years ago, back when we had the 640K barrier, heck, you could just throw all your code away every year and rewrite it. The rewrite took about a year, and that coincided with the next release. So you didn't really have to worry about reusing the old stuff. Versioning? What's that? We started from scratch every time.

I hate to break it to you, but that's gone. We will never catch up. We will never fill all the capacity that Moore's law is heaping on us. The way we get more functionality these days is by doing more and more leveraging of existing infrastructure and existing applications. As systems are becoming longer lived, versioning is becoming more important.

Next Week

Come back Monday, September 22 for part II of a conversation with Java's creator James Gosling. I am now staggering the publication of several interviews at once, to give the reader variety. The next installment of this interview with Anders Hejlsberg will appear on Monday, September 29. If you'd like to receive a brief weekly email announcing new articles at Artima.com, please subscribe to the Artima Newsletter.

Talk Back!

Have an opinion about the design principles presented in this article? Discuss this article in the News & Ideas Forum topic, Versioning, Virtual, and Override.

Resources

Deep Inside C#: An Interview with Microsoft Chief Architect Anders Hejlsberg:
http://windows.oreilly.com/news/hejlsberg_0800.html

A Comparative Overview of C#:
http://genamics.com/developer/csharp_comparative.htm

Microsoft Visual C#:
http://msdn.microsoft.com/vcsharp/

Anders Hejlsberg was not the first Artima interviewee to mention taste. Jim Waldo made almost an identical comment about building a team of tasteful programmers in his interview:
http://www.artima.com/intv/waldo10.html

And an entire portion of Ken Arnold's interview was devoted to design taste - Taste and Aesthetics:
http://www.artima.com/intv/taste.html