title: Expiring Concurrent Dictionary
body: <p>Hi I'm doing some research after a concurrent dictionary with expiration features.</p>
  <p>We have a list of objects that are accessed with adds and removes from a lot different threads.
  We also want an expiration time on the items for like lets say 60 secconds.
  (It would be nice if we can listen on an event that tells us which items that has been expired and removed from the list).</p>
  <p>We also need the lookup to be really fast like an Dictionary.</p>
  <p>The list will contain houndred of thousands objects.</p>
  <p>So its like an ConcurrentDictionary but with expiration features.</p>
  <p>Can MemoryCache be something?</p>
  <p>Or are they other things to look on?</p>