Session Expire ??

hello guys how r u ??

my question is that, i have created a web application and in that web app i check session["userinfo"] not null on every page  on page_load event. but the problem is that, if user stay on any page and he went for any other work and then he came back after 30 or 45 min and he try to use my web app again and he clicked a button to view report so session expire exception occured or some thing like this ...  runtime error occured ...


reply soon  how can i come over to this 


Thanks