John Rusk on Extension Interfaces

Sorry for not blogging recently, I was ramping up and getting started in my new role on the C# team. I love it here! The people are great, the job is interesting and I'm really proud to work in C#. There are a lot of interesting things starting here, like F# or other very exciting projects I can't talk about at this point ;-)


I posted recently about duck-typing and extension interfaces. John Rusk pointed me to his great article about extension interfaces. It was posted back in 2006 but I only discovered it recently (sorry). It was interesting to see Keith Farmer's comments on the article, especially because I'm now sitting two offices down the hall from him so I could go and ask him what he thinks about it in person. But I digress.


I like John's ideas and considerations, and I currently can't think of any caveats of implementing extension interfaces from the design point of view. From the implementation perspective, there might be a complication John warns about. To make an existing class implement an interface, you'd usually write an adapter class. Now the problem is: a reference to the adapter object won't be the same with a reference to the actual target object, and that might become a problem, e.g. when you'd like to compare objects. Being constructive, John also points to a possible solution involving CLR internal special cases like TransparentProxy (see an as-always-excellent post by Chris Brumme about it).

1 comment:

John Rusk said...

Hi Kirill.

Thanks for your comments on the idea. I've also exchanged a few blog comments on this subject with Eric Lippert, here: http://blogs.msdn.com/ericlippert/archive/2007/09/04/future-breaking-changes-part-three.aspx

You comments, and his, got me thinking: could extension interfaces be safely introduced in a future version of Visual Studio, now that VS 2008 has almost shipped without them? I believe they could, as I've just noted here: http://dotnet.agilekiwi.com/blog/2007/11/making-extension-methods-safer.html