How do I clear a System.Runtime.Caching.MemoryCach

问题:

I use a System.Runtime.Caching.MemoryCache to hold items which never expire. However, at times I need the ability to clear the entire cache. How do I do that?

I asked a similar question here concerning whether I could enumerate the cache, but that is a bad idea as it needs to be synchronised during enumeration.

I've tried using .Trim(100) but that doesn't work at all.

I've tried getting a list of all the keys via Linq, but then I'm back where I started because evicting items one-by-one can easily lead to race conditions.

I thought to store all the keys, and then issue a .Remove(key) for each one, but there is an implied race condition there too, so I'd need to lock access to the list of keys, and things get messy again.

I then thought that I should be able to call .Dispose() on the entire cache, but I'm not sure if this is the best approach, due to the way it's implemented.

Using ChangeMonitors is not an option for my design, and is unnecassarily complex for such a trivial requirement.

So, how do I completely clear the cache?

回答1:

You should not call dispose on the Default member of the MemoryCache if you want to be able to use it anymore:

The state of the cache is set to indicate that the cache is disposed. Any attempt to call public caching methods that change the state of the cache, such as methods that add, remove, or retrieve cache entries, might cause unexpected behavior. For example, if you call the Set method after the cache is disposed, a no-op error occurs. If you attempt to retrieve items from the cache, the Get method will always return Nothing. http://msdn.microsoft.com/en-us/library/system.runtime.caching.memorycache.dispose.aspx

About the Trim, it's supposed to work:

The Trim property first removes entries that have exceeded either an absolute or sliding expiration. Any callbacks that are registered for items that are removed will be passed a removed reason of Expired.

If removing expired entries is insufficient to reach the specified trim percentage, additional entries will be removed from the cache based on a least-recently used (LRU) algorithm until the requested trim percentage is reached.

But two other users reported it doesnt work on same page so I guess you are stuck with Remove() http://msdn.microsoft.com/en-us/library/system.runtime.caching.memorycache.trim.aspx

Update However I see no mention of it being singleton or otherwise unsafe to have multiple instances so you should be able to overwrite your reference.

But if you need to free the memory from the Default instance you will have to clear it manually or destroy it permanently via dispose (rendering it unusable).

Based on your question you could make your own singleton-imposing class returning a Memorycache you may internally dispose at will.. Being the nature of a cache :-)

回答2:

I was struggling with this at first. MemoryCache.Default.Trim(100) does not work (as discussed). Trim is a best attempt, so if there are 100 items in the cache, and you call Trim(100) it will remove the ones least used.

Trim returns the count of items removed, and most people expect that to remove all items.

This code removes all items from MemoryCache for me in my xUnit tests with MemoryCache.Default. MemoryCache.Default is the default Region.

foreach (var element in MemoryCache.Default)
{
    MemoryCache.Default.Remove(element.Key);
}

回答3:

Here's is what I had made for something I was working on...

public void Flush()
{
    List<string> cacheKeys = MemoryCache.Default.Select(kvp => kvp.Key).ToList();
    foreach (string cacheKey in cacheKeys)
    {
        MemoryCache.Default.Remove(cacheKey);
    }
}

回答4:

The details in @stefan's answer detail the principle; here's how I'd do it.

One should synchronise access to the cache whilst recreating it, to avoid the race condition of client code accessing the cache after it is disposed, but before it is recreated.

To avoid this synchronisation, do this in your adapter class (which wraps the MemoryCache):

public void clearCache() {
  var oldCache = TheCache;
  TheCache = new MemoryCache("NewCacheName", ...);
  oldCache.Dispose();
  GC.Collect();
}

This way, TheCache is always in a non-disposed state, and no synchronisation is needed.

回答5:

I ran into this problem too. .Dispose() did something quite different than what I expected.

Instead, I added a static field to my controller class. I did not use the default cache, to get around this behavior, but created a private one (if you want to call it that). So my implementation looked a bit like this:

public class MyController : Controller
{

    static MemoryCache s_cache = new MemoryCache("myCache");

    public ActionResult Index()
    {

        if (conditionThatInvalidatesCache)
        {
            s_cache = new MemoryCache("myCache");
        }

        String s = s_cache["key"] as String;

        if (s == null)
        {
            //do work
            //add to s_cache["key"]
        }

        //do whatever next
    }
}

回答6:

Check out this post, and specifically, the answer that Thomas F. Abraham posted. It has a solution that enables you to clear the entire cache or a named subset.

The key thing here is:

// Cache objects are obligated to remove entry upon change notification.
base.OnChanged(null);

I've implemented this myself, and everything seems to work just fine.