Getting custom CacheItems from MemoryCache

Question

I'm implementing a MemoryCache with a derived CacheItem, but having difficulty interacting with it once it's in the cache. For example:

class Program
{
    static void Main(string[] args)
    {
        MemoryCache cache = MemoryCache.Default;
        CacheItemPolicy policy = new CacheItemPolicy();
        CustomCacheItem someItem = (CustomCacheItem)cache.AddOrGetExisting(new CustomCacheItem(1, "tacos", "waffles"), policy);

        Console.ReadLine();
    }
}

public class CustomCacheItem : CacheItem
{
    public int FailureCt { get; set; }

    public CustomCacheItem(int _failureCt, string _key, string _value)
        : base(_key, _value)
    {
        FailureCt = _failureCt;
    }
}

This throws an error of Unable to cast object of type 'System.Runtime.Caching.CacheItem' to type 'CacheTest.CustomCacheItem'. which sort of makes sense; maybe it doesn't retain the information about the cache item put in. But if so, how do I get my custom cache item out? How do I interact with that property (in this case FailureCt) if the return value is of the generic base type?

Answer 1

The reason is that MemoryCache.AddOrGetExisting(CacheItem, CacheItemPolicy) is creating a new CacheItem internally:

public override CacheItem AddOrGetExisting(CacheItem item, CacheItemPolicy policy)
{
    if (item == null)
        throw new ArgumentNullException("item");
    return new CacheItem(item.Key, AddOrGetExistingInternal(item.Key, item.Value, policy));
}

MemoryCache source code

---

I'd recommend storing FailureCt in the value itself and not in the CacheItem wrapper:

public class CacheValue
{
    public int FailureCt { get; set; }
    public string Value { get; set; }
}

And then:

CacheValue someItem = (CacheValue)cache.AddOrGetExisting("tacos", new CacheValue()
{
    FailureCt = 1,
    Value = "waffles"
}, policy);