There is a lock on the toilet door in the dormitory. Do I have to knock before I go to the toilet?

Sure, if you want to knock. This is a virtue handed down from China. She's right, thanks to the lock. Otherwise, the scene will be awkward. If it's outside in the public toilet, so are you. People you don't know are probably ready to scold you. As long as the door is closed. Knocking at the door is basic courtesy. In case something happens to others. Or inconvenient for others to disturb. You suddenly went in and interrupted someone. Or influence others. Embarrassment can also bring trouble to others.