prettycode.org

Expiring Cache in C#

Posted October 23, 2009 by Chris | Filed under .NET, C#

Ever need a simple cache that automatically expires items, removing them after a given time has elapsed? Well now you've got it: ExpiringValueCache and ExpiringObjectCache. ExpiringValueCache is used to store value types. ExpiringObjectCache is used to store reference types.

A background thread continuously runs, checking for expired items in a generic Dictionary. The size of the collection does not affect the productivity of this background expiration process. The scalability of the caches is the same as that of a generic Dictionary of the items that you're storing.

First, to create one of these expiring caches, pass in a TimeSpan for which items should live before being automatically removed. All objects in the cache use the same TimeSpan as thier lifespan. Here are caches with one-hour expirations:

var peopleCache = new ExpiringObjectCache(new TimeSpan(1, 0, 0));

var idCache = new ExpiringValueCache(new TimeSpan(1, 0, 0));

Next, to add an item to the cache, call its Add method. The first argument is the key for the item, and the second argument is the item.

peopleCache.Add("chris", people[0]);

idCache.Add("chris", people[0].Id);

To check for the existence of an item or retrieve it, use the cache collection's indexer. For ExpiringObjectCache, the object will be returned if the item exists; null if it's not or no longer (i.e. it expired) in the cache:

var chris = peopleCache["chris"];
 
if (chris == null)
{
    // Not in cache. Expired?
}
else
{
    // In cache, do something...
}

Use ExpiringValueCache the same way—it also returns null if the item doesn't exist. It does this by wrapping the result in a System.Nullable:

var chrisId = idCache["chris"];
 
if (chrisId == null)
{
    // Not in cache. Expired?
}
else
{
    var id = chrisId.Value;
}

Lastly, since the caches have a background thread expiring stale items, remember to Dispose() of the cache when you're done with it. The thread is a background thread, so it won't prevent the process from terminating, but you should always cleanly dispose of your resources.

That's it. Suitably simple, I hope.

Permalink | Comments (1) | October 23, 2009

One Response to “Expiring Cache in C#”

1. Marc Schubert Says:
   November 26th, 2009 at 4:18 pm

   Very nice; simular in ASP.NET-Cache.
   Acutally i solve such problems with two dictionaries and doing an manual look-up if the objects are in an expired state.

   Regards,

   Marc

Leave a Reply