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

Question

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?

Answer 1

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);
}

Answer 2

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);
    }
}

Answer 3

I know this is an old question but the best option I've come across is to

Dispose the existing MemoryCache and create a new MemoryCache object. https://stackoverflow.com/a/4183319/880642

The answer doesn't really provide the code to do this in a thread safe way. But this can be achieved using Interlocked.Exchange

var oldCache = Interlocked.Exchange(ref _existingCache, new MemoryCache("newCacheName"));
oldCache.Dispose();

This will swap the existing cache with a new one and allow you to safely call Dispose on the original cache. This avoids needing to enumerate the items in the cache and race conditions caused by disposing a cache while it is in use.

---

Edit

Here's how I use it in practice accounting for DI

public class CustomCacheProvider : ICustomCacheProvider
{
    private IMemoryCache _internalCache;
    private readonly ICacheFactory _cacheFactory;

    public CustomCacheProvider (ICacheFactory cacheFactory)
    {
        _cacheFactory = cacheFactory;
        _internalCache = _cacheFactory.CreateInstance();
    }
    public void Set(string key, object item, MemoryCacheEntryOptions policy)
    {
        _internalCache.Set(key, item, policy);
    }

    public object Get(string key)
    {
        return _internalCache.Get(key);
    }
   // other methods ignored for breviy 

    public void Dispose()
    {
        _internalCache?.Dispose();
    }

    public void EmptyCache()
    {
        var oldCache = Interlocked.Exchange(ref _internalCache, _cacheFactory.CreateInstance());
        oldCache.Dispose();
    }
}

The key is controlling access to the internal cache using another singleton which has the ability to create new cache instances using a factory (or manually if you prefer).

Answer 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.

Answer 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
    }
}

Answer 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.