counting online number of users of a web application

I am using ASP.Net + .Net 3.5 + VS2008 + IIS 7.0 + C# to develop a web application. I want to count how many users are online. This is my current implementation,

1. when Session_Start is called, I will increase # of users online by 1;
2. when Session_End is called, I will decrease # of users online by 1.

Two questions,

A Is that implementation correct?

B. Another question is, I think this method can not track # of users of real t开发者_StackOverflowime, since when user closes the browser, Session_End will not be called immediately (Session_End will be deferred to be called). Correct?

Are you using ASP.NET membership?

Membership.GetNumberOfUsersOnline()

You can customise the amount of time a user is considered to be "online" in web.config - eg

<membership defaultProvider="..." 
            userIsOnlineTimeWindow="10" 
  <providers>
    ....
  </providers>
</membership>

Read more here

Another problem with using Session_End is that it does not fire reliably, in fact if you use anything but inproc session state you cannot rely on this at all.

A better option is to use the the Membership provider, take a look at Membership.GetNumberOfUsersOnline()

Your implementation won't be very accurate - if a user logs on using two browsers your counter will increase by two.

A lot of sites just record logons and then do a count of the number of unique logons in the last x minutes to get an online total.