In one of the recent posts in the TDD chess series, I was asked a question that made me sort of stop and muse a little after it was asked. It had to do with whether to define a custom collection type or simply use an out of the box one and my thinking went, “well, I’d use an out of the box one unless there were some compelling reason to build my own.” This got me to thinking about something that had gone the way of the Dodo in my coding, which was to build collection types by inheriting from existing collections or else implementing collection interfaces. I used to do that and now I don’t do that anymore. At some point, it simply stopped happening. But why?