Bill Henry makes a valid point about the cassock being everyday clothing – and I would argue that properly speaking, it requires that no clothing be worn underneath.
While many clerics wear their cassocks in church, but not on the street, I recall a rule at our synods that clergy should wear cassocks during synod session – which ought to suggest that they should not wear trousers as well!
On the other hand, a chasuble is not the same as a cassock, and is rarely seen on the streets (except in processions).

Dale, your picture shows that you were very well dressed indeed, and must have put those trouser wearers to shame!
Regards,
Mike