.NET Caching how does Sliding Expiration work?

Question

If I use an ObjectCache and add an item like so:

ObjectCache cache = MemoryCache.Default;
string o = "mydata";
cache.Add("mykey", o, DateTime.Now.AddDays(1));

I understand the object will expire in 1 day. But if the object is accessed 1/2 a day later using:

object mystuff = cache["mykey"];

Does this reset the timer so it's now 1 day since the last access of the entry with the key "mykey", or it still 1/2 a day until expiry?

If the answer is no is there is a way to do this I would love to know.

Answer 1

There are two types of cache policies you can use:

CacheItemPolicy.AbsoluteExpiration will expire the entry after a set amount of time.

CacheItemPolicy.SlidingExpiration will expire the entry if it hasn't been accessed in a set amount of time.

The ObjectCache Add() overload you're using treats it as an absolute expiration, which means it'll expire after 1 day, regardless of how many times you access it. You'll need to use one of the other overloads. Here's how you'd set a sliding expiration (it's a bit more complicated):

CacheItem item = cache.GetCacheItem("item");

if (item == null) {

    CacheItemPolicy policy = new CacheItemPolicy {
        SlidingExpiration = TimeSpan.FromDays(1)
    }

    item = new CacheItem("item", someData);

    cache.Set(item, policy);
}

You change the TimeSpan to the appropriate cache time that you want.