how to prevent another user from login if one user is access

There is a perfect way while using flag to detect whether someone is already logged in or not and in case if System get power off , you need an event handler called as SystemEvents.SessionEnds.

 

Use this to change the flag value from true to false

 

Use this link as a reference

https://stackoverflow.com/questions/6799955/how-to-detect-windows-shutdown-or-logoff 

Hey Ajeesh

 

one more question... ??

 

If power off happen then how you suppose to Reset the flag

You can take a look at this article which discuss a similar topic. 

 

https://blog.learningtree.com/preventing-duplicate-logins-in-asp-net/

 

Another option would be to have a flag in your table and set the value to true when an user login and when user log off set the value to false.  Also for when a second user tries to login we will first check if login flag has been set to true for any other user. If its true then show message to user and display the login page, if not allow the user to login.