Is it haram to pray in a dancing hall

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

First, you should have asked about the ruling on going to discos (dancing halls) before asking about the ruling on praying there! Being present in places of evil and immorality is not permissible except for a legitimate dire need, such as helping a sick person, or enjoining good and forbidding evil.

With regard to praying in these places, it is disliked.

Imam Nawawi said in Kitab Al-Majmoo’:

“Praying in the abode of Satan is disliked according to the consensus of the scholars, such as places of wine [all kinds of intoxicants] and bars… and other outrageous acts of disobedience and immorality…if he prays in any such places and does not touch impurity with his hand or his clothes, then his prayer is valid but it is disliked.” [End of quote]

Based on the above, it is not the prayer in this place that is forbidden, but it is being present there that is forbidden if it is not for a dire need, let alone practicing evil.

Allah Knows best.