C# Collection whose items expire

Question

I am writing a Console Application in C# in which I want to cache certain items for a predefined time (let's say 1 hour). I want items that have been added into this cache to be automatically removed after they expire. Is there a built-in data structure that I can use? Remember this is a Console App not a web app.

Answer 1

I don't know of any objects in the BCL which do this, but I have written similar things before.

You can do this fairly easily by just including a System.Threading.Timer inside of your caching class (no web/winforms dependencies), and storing an expiration (or last used) time on your objects. Just have the timer check every few minutes, and remove the objects you want to expire.

However, be watchful of events on your objects. I had a system like this, and was not being very careful to unsubscribe from events on my objects in the cache, which was preventing a subtle, but nasty memeory leak over time. This can be very tricky to debug.

Answer 2

Do you actually need them removed from the cache at that time? Or just that future requests to the cache for that item should return null after a given time?

To do the former, you would need some sort of background thread that was periodically purging the cache. This would only be needed if you were worried about memory consumption or something. If you just want the data to expire, that would be easy to do.

It is trivial to create such a class.

class CachedObject<TValue> 
{ 
 DateTime Date{get;set;}
 TimeSpan Duration{get;set;}
 TValue Cached{get;set;}
}

class Cache : Dictionary<TKey,TValue>
{
  public new TValue this(TKey key)
  {
    get{
    if (ContainsKey(key))
    {
       var val = base.this[key];
       //compare dates
       //if expired, remove from cache, return null
       //else return the cached item.
    }
    }

    set{//create new CachedObject, set date and timespan, set value, add to dictionary}

  }

Answer 3

Include an ExpirationDate property in the object that you will be caching (probably a wrapper around your real object) and set it to expire in an hour in its constructor. Instead of removing items from the collection, access the collection through a method that filters out the expired items. Or create a custom collection that does this automatically. If you need to actually remove items from the cache, your custom collection could instead purge expired items on every call to one of its members.

Answer 4

Its already in the BCL. Its just not where you expect to find it: You can use System.Web.Caching from other kinds of applications too, not only in ASP.NET.

This search on google links to several resources about this.