@treyhard thanks for the added info, I understand what's going on now.

First, a timeout exception is an NpgsqlException and not a PostgresException because it's generated by Npgsql, and not by PostgreSQL. In 3.1, timeouts are implemented as client-side socket timeouts - Npgsql itself gives up on the response, rather than PostgreSQL returning a timeout error (as was the case before 3.0).

The BaseMessage property is obsolete as marked - PostgresException.MessageText should be used instead. However, BaseMessage was left on NpgsqlException for compatibility with Hangfire. Since it internally casts the exception to PostgresException, this will throw an InvalidCastException if the exception is actually an NpgsqlException. This was just a backwards-compatibility compromise, it seems that Hangfire have updated their code so I'll be removing this property in the upcoming Npgsql 3.2.

Long story short - timeouts are a client-side error, so they're not a PostgresException. Avoid BaseMessage in any case.