Hi,
Currently I am using vb.net to create a web application. I would like to pop up a confirmation message box to the user 1 minutes before the session end to ask whether the user would like to continue with the session. So, how could I detect the time before the session end? and is there any way to pop up the confirmation message box with pure vb.net code without using javascript?

You cannot know beforehand that the Session is going to be dropped—at least, not from the Session itself. You would need to find the Session’s timeout setting, and calculate for yourself—independently—how close it is to expiring.
I don’t think you are going to be able to do this at the server, because any query of the server will reset the time-expired to 0.
You can send the timeout value to the web page, and have the page count off the minutes, but—as you suspect—that will take some client-side processing.