Rav Shlomo Zalman Auerbach says that the Medakdikim are noheg not to eat before counting Sefiras HaOmer. Moreover he says, you may not eat and rely on a Shomer to remind you, like you may do by Maariv (Halichos Shlomo Sefiras HaOmer 11:1). The Orchos Halacha (Halichos Shlomo footnote 1) brings from the Yosef Ometz on Minhagei Frankfurt that even though technically you may continue eating if you started before the zman, nevertheless experience shows that this is a cause for many mishaps, and since so much is at stake since the bracha each night is part of one complete mitzva, it is better to stop the meal and count Sefira.

Furthermore, says the Orchos Halacha, one time Rav Shlomo Zalman told a Yeshiva bochur, who davened with a late minyan for maariv in Yeshiva, that he should count without a bracha as soon as it becomes night and have in mind that if he only wants to be yotzei with this counting if forgets to count later on during Maariv with a bracha.

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