Dreaming of affair is considered negative and indication of future problem as per dream interpretation and meaning. You need to be careful with all your interactions after this dream.
Dream of affair with someone you know means you can expect someone you personally know to create problems in life. Your relatives or friends might turn enemies.
Dream of affair with someone you do not know means there will be unexpected problems with strangers. You need to be careful while traveling and avoid travel at night and to unknown places.
Dream of affair and you are sad or tensed means an action of yours will bring bad name and problems.
Dream of affair and you are happy means you will be taken for a ride by someone. They will be friendly but their real intention will be to cheat you.