Hi, Zurich is pretty casual for a large financial center but I would doubt that gentlemen attend the opera in the evening in sweaters even with a tie. Hopefully, someone who attends the opera in Zurich will reply soon as I do not but am judging on my experience with other events in Zurich and with London dress for similar activities.
Hope this helps!
I don't know specifically about the opera but for classical concerts in general, even jeans are fine.
At the risk of being provocative, people who feel the need to dress up to go to a concert are often there because it's the place to be seen more than for the music... which is the last thing that classical music needs!
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