He's right, sorry for that. There's nothing wrong with saying it though. Nowadays the Genitiv is slowly disappearing and being replaced by Dativ in common speech.I agree, just one comment: the third example would be "ich singe während des Duschens" in written language (Genitiv instead of Dativ)
Yes you're right.Ich singe während des Essens < Am I right?
I agree: even though genitive in many phrases is replaced by dative in colloquial speech, here "während dem Duschen" does not sound colloquial at all - while "während des Duschens" is perfectly okay for written language but rather unusual for spoken language.With all due respelt, that might not be true for all areas where German is spoken.
"Ich singe während dem Duschen." would certainly stand out among my fellows.