It was during a presentation on the NDC developer conference a few years back, when the feature was new to C#. I remember thinking it was one of the best presentations that year, but right now I don't remember what year it was, and I can't find any past agendas to search either.

NDC posts their videos online after each conference, and I did a quick skim. I think it may be this session here, but I haven't double-checked:

https://vimeo.com/43808831

Edit: It was a 2-part session and it may also have been in second half. Link here:

https://vimeo.com/43808833