Doubt Session Asp.net

I need to pass the id of a user by a session, as it is happening to mix users, since it was being written to a variable, has some way, to continue passing through the variable without having to change throughout the project, because in every project it is already being picked up by the class variable. In case any way to pass the session to variable only? I tried going straight from the default, but as it is changing to the variable, without any verification, it is still mixing.