Access Facebook UserId in Facebook Canvas before OAuth Authentification

In my facebook canvas applikation, i store the access token in session. (I use ASP.NET MVC)

But i want to store the access token in a database along with the facebook user id that i can save different access tokens for different facebook users accessing my application.

So when i query the facebook user id and if it doen't return any access token, i request permissions and get the access token for that user.

How do i access the Facebook UserID for the user accessing my canvas application before d开发者_开发知识库oing any authentification?

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You can't access FB user id if the user doesn't grant access. Imagine that it is allowed without permission, you can access basic information about the user, and that makes the permission meaningless.