I have found some similar questions but none gave me what I really need.
Here is the thing, I have added this to my web.config to handle user session ex
Session_End is a server-side event, meaning it is triggered on the web server and has nothing to do with a request by the client. This is why the Request is unavailable.
You have two choices in this matter:
On each client request, check if a specific Session variable is set. If it is not, it means the previous Session has expired and the new Session must be populated. (I am assuming this is why you want to check for Session expiration)
Have a javascript call on the client that periodically goes back to the server to check if the Session is still valid. If the Session has expired, you can redirect the user to the login page.
samples of different redirect methods
location.href = "login.aspx";
// or you can use
location.assign("login.aspx");
//for redirecting without storing in history
location.replace("login.aspx")
Don't forget to add ?ReturnUrl=[current url] to the login redirect path.
HTH