I'm not clear on what you GetLines() method is returning (thanks to the var keyword - very useful but sometimes problematic, this is one of those times). If yo

I'm not clear on what you GetLines() method is returning (thanks to the var keyword - very useful but sometimes problematic, this is one of those times). If your GetLines() is returning a string[] then I agree there was no benefit. But that's not what I meant. As I said, I'm imagining the base class (ILinesProvider) to have an iterator-like interface. So, rather than string[] GetLines() method, it would have a string getNextLine() method that you call in a loop. That way, with the file, you don't load the whole thing in memory (as I also said). If your GetLines() method is returning an IEnumerable<string> (since this seems to be C#) then this is the problem I mentioned at the end of my comment - the base class I'm imagining is similar to an existing base class in the language, and it's confusing to write your own similar-but-slightly-different version in an example. But I think it's best to do it anyway and explain it away in a footnote. (But my ILinesProvider wouldn't return an IEnumerable, like in your snippet - instead, ILinesProvider actually is the iterator (but with a slightly different interface).)