Does accessing MemoryCache create a copy?

Question

I have a cache service like this:

public interface ICacheService {
    T Get<T>(string cacheID, Func<T> getItemCallback, int cacheMinutes = 5) where T : class;
}

public class MemoryCacheService : ICacheService {
    public T Get<T>(string cacheId, Func<T> getItemCallback, int cacheMinutes = 5) where T : class {
        T item = MemoryCache.Default.Get(cacheId) as T;
        if (item == null) {
            item = getItemCallback();
            MemoryCache.Default.Add(cacheId, item,
                new CacheItemPolicy {AbsoluteExpiration = DateTime.Now.AddMinutes(cacheMinutes)});
        }
        return item;
    }
}

And retrieved like this:

var result = _cache.Get("mylist", () => _database.Fetch<MyList>().AsQueryable(), 600);

The list is large and accessed frequently in a per keystroke type-ahead dropdown. And the query condition is also dynamic, like

if (this) result = result.Where(x=> this ...)
if (that) result = result.Where(x=> that ...)
finally result.ToList() 

I wonder, every time I access the list from cache, does the system create a copy of the data before start building linq query? If so, it's like copy-per-keystroke, not very efficient. Or does the it deferred the query because I'm retrieving AsQueryable and build linq?

Any better alternatives? Thanks

Answer 1

Without getting lost in the minutia of MemoryCache, you can reason this out with basic .NET design principles. Only value types are easy to copy. There is no general mechanism to copy reference types, beyond [Serializable] and the very broken ICloneable. Which are not requirements for an object to be put in the MemoryCache. So no.

Caching objects is very, very simple. A simple List<> gets that job done. The value-add you get from MemoryCache is the other essential feature of an effective cache. A retirement policy. A cache without a policy is a memory leak.

Answer 2

No, MemoryCache does not make a copy. You basically store a reference to some object instance in the cache, and that is what you get back when you access an item in the cache.

I don't have a formal documentation link, but found out the "hard way" in practice, where I accidentally modified the cached object by just using the reference I got back (without copying it).

Also, studying the reference sources (http://referencesource.microsoft.com) shows that there is no automatic copying happening.

Depending on your application and needs, you might to want sure that the types you cache are actually immutable by design.