CODEWITHSUNDEEP
c#.netcaching
Andres
Andres

Reputation: 687

Combine Sliding and Absolute Expiration

I want to use System.Runtime.Caching.MemoryCache for caching some of my objects. I want to be sure that the object is refreshed once a day (absolute expiration) but I also want to make it expire if it hasn't been used in the last hour (sliding expiration). I try to do:

object item = "someitem";
var cache = MemoryCache.Default;
var policy = new CacheItemPolicy();
policy.AbsoluteExpiration = DateTime.Now.AddDays(1);
policy.SlidingExpiration = TimeSpan.FromHours(1);
cache.Add("somekey", item, policy);

But I'm getting an error:

"ArgumentException" with "AbsoluteExpiration must be DateTimeOffset.MaxValue or SlidingExpiration must be TimeSpan.Zero."

Upvotes: 22

Views: 15069

Answers (5)

Uwe
Uwe

Reputation: 316

A little additional notes to dieguitofernandez answer (which is btw, a very good solution):

The ChangeMonitor isn't fiering at the exact moment the AbsoluteExpiration is reached, because the Expiration isn't checked every second. It may take several seconds until the second cached item is notified. With 1 day as AbsoluteExpiration it doesn't matter, but if you need both expirations to be exact (like i did) you have to use both keys and the absolut-key first.

if (cache.Contains("Absolute" + key) && cache.Contains(key))

This way both expirations are checked immediately and you can be sure the item isn't expired.

(I could post this as comment because I'm new here, sorry!)

Upvotes: 2

dieguitofernandez
dieguitofernandez

Reputation: 116

You can implement both schemes cache expiration by using CacheEntryChangeMonitor. Insert a cache item without information with absolute expiration, then create a empty monitorChange with this item and link it with a second cache item, where you will actually save a slidingTimeOut information.

        object data = new object();
        string key = "UniqueIDOfDataObject";
        //Insert empty cache item with absolute timeout
        string[] absKey = { "Absolute" + key };
        MemoryCache.Default.Add("Absolute" + key, new object(), DateTimeOffset.Now.AddMinutes(10));

        //Create a CacheEntryChangeMonitor link to absolute timeout cache item
        CacheEntryChangeMonitor monitor = MemoryCache.Default.CreateCacheEntryChangeMonitor(absKey);

        //Insert data cache item with sliding timeout using changeMonitors
        CacheItemPolicy itemPolicy = new CacheItemPolicy();
        itemPolicy.ChangeMonitors.Add(monitor);
        itemPolicy.SlidingExpiration = new TimeSpan(0, 60, 0);
        MemoryCache.Default.Add(key, data, itemPolicy, null);

Upvotes: 10

Charles Lambert
Charles Lambert

Reputation: 5132

You have two options:

  1. Inherit from MemoryCache or it's parent ObjectCache and implement your own caching mechanism.
  2. Implement a ChangeMonitor which may be more difficult (see Note to Inheritors)

Upvotes: 2

Cybermaxs
Cybermaxs

Reputation: 24558

A quick reflection with ILSpy show this code when calling MemoryCache.Add

if (policy.AbsoluteExpiration != ObjectCache.InfiniteAbsoluteExpiration && policy.SlidingExpiration != ObjectCache.NoSlidingExpiration)
    {
        throw new ArgumentException(R.Invalid_expiration_combination, "policy");
    }

So, this combination of Absolute and Sliding Expiration is not supported natively.

You should turn yourself to a custom implementation.

Upvotes: 5

ThaMe90
ThaMe90

Reputation: 4296

DateTime.Now.AddDays(1) doesn't return a DateTimeOffset, but a DateTime (AddDays()).

So it's most likely an invalid argument.

So instead, try DateTimeOffset.Now.AddDays(1) (DateTimeOffset).

Upvotes: -2

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