Background I've been to a few talks on SOLID before. Most of the principles seem pretty reasonable to me – but I've never "got" the open-closed principle (OCP from here on). At CodeMash this year, I mentioned this to the wonderful Cori Drew, who said that she'd been at a user group talk where she felt it was explained well. She mailed me a link to the user group video, which I finally managed to get round to watching last week. (The OCP part is at around 1 hour 20.) Unfortunately I still wasn't satisfied[...]