Session_End is called multiple times in the middle of the session

Am I using Session_End incorrectly?

In my application I have a cache to make the pages load faster, it works very well. Someone recently suggested to me that I would leak memory by never emptying it, and implement a method on Session_End that will clear any cache entries associated with that session ID.

Sometimes I started to see abnormal behavior. On page load, it is very rare that I see a blank page showing an exception due to the cache for that page being empty. I put a debugger breakpoint in cache access mode and session_end methods, hoping to catch it the next time it occurs.

The next time this happened, I was very surprised at what was actually happening. My browser was left open but idle for a long time, so I am assuming it had a stale session cookie. It hits the server for the page, which puts a copy of the page in the cache. While the page is still loading, Session_End is called instantly, which removes it from the cache. If I restart my browser, Session_End keeps getting calls, aggressively trying to close my session, which the poor browser is asking for.

So, I suppose my real questions are:

• Why doesn't the browser get the new session cookie when the old one is anathema to IIS?
• Is Session_End just not meant to be used this way?
• Is there a better way to do what I am trying to do?

@Sosh - my global.asax.cs:

    protected void Session_End(Object sender, EventArgs e)
    {
        //clean up object cache on session expiry
        string sessionID = this.Session.SessionID;

        foreach (string key in Global.DataGroups.Keys)
        {
            if (key.EndsWith(sessionID))
            {
                Global.DataGroups.Remove(key);
            }
        }
    }