Active code for login user by time in asp.net core?

David's advice is very good, you should write the code first and ask questions when you encounter problems.

Regarding your needs, I can provide you with some ideas.

1. First you need to know that when the browser client connects with the asp.net core webserver, the session ID is generated when the connection is established.

2. We can customize the code, codeState in session, to determine whether the user who established the connection has logged in.

   code            string type: 0123 or Az02
   
   codeState       null ->  not generated
                   -1   ->  expired
                   0    ->  generated but un-use
                   1    ->  used
   

   ---

   if(codeState=null){
       code = generate_code();
       HttpContext.Session.SetString("code",code);
   }
   if(codeState == -1){
       msg = "The code has expired, pls click the button to generate a new code";
   }
   if(codeState== 0){
       msg = "The code has generated";
       code = HttpContext.Session.GetString("code");
   }
   if(codeState== 1){
       msg = "The code has used";
   }