If the cops come to my dorm room on suspicion of marijuana possession, do I have to open my door when they knock? Do I have to open locked containers if they ask me to?

In theory, you don't have to open the door, but the police don't always honor your constional rights under the Fourth Amendment, to be free of unreasonablesearches or seizures. The only way the cops can enter your room is with a search warrant issued under penalty of perjury. But often the cops will bust the door down and say there were "exigent circumstances" that would excuse the warrant.  As for the "closed containers," they should have a search warrant for that also, but they may say they don't need one because it's an "inventory" inspection. They have hundreds of excuses and pretexts to avoid playing by the rules.  Also, you don't have to even talk to the police or anyone else in law enforcement.  There are millions of cases involving searches and seizures under the Fourth Amendment.

I hope this information is helpful to you.