Detect expiration in Web.Caching.Cache on a webservice

[nmpgaspar]

Hey.

I'm using System.Web.Caching in my .asmx webservice in order to save some information about multiple users requests.

I insert that data like this:
cache.Insert(Unique_token, MySessionObjectForThatUserToken, null, Cache.NoAbsoluteExpiration, new TimeSpan(0, 15, 0));

And then retrieve it like this:
MySessionObjectForThatUserToken obj = (MySessionObjectForThatUserToken)cache[token];

As you can see, i have a slidding expiration time of 15minutes. So now, what i wanna know, is if it's possible to have some event that will fire whenever an object gets expired from the cache?

How can i archive this?
I would just need to know, what is the 'token' associated with the expired object.