MemoryCache AbsoluteExpiration acting strange

I’ve figured it out. There’s an internal static readonly TimeSpan on System.Runtime.Caching.CacheExpires called _tsPerBucket that is hardcoded at 20 seconds.

Apparently, this field is what’s used on the internal timers that run and check to see if cache items are expired.

I’m working around this by overwriting the value using reflection and clearing the default MemoryCache instance to reset everything. It seems to work, even if it is a giant hack.

Here’s the updated code:

static MemoryCache MemCache;
static int RefreshInterval = 1000;

protected void Page_Load(object sender, EventArgs e)
{
    if (MemCache == null)
    {
        const string assembly = "System.Runtime.Caching, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a";
        var type = Type.GetType("System.Runtime.Caching.CacheExpires, " + assembly, true, true);
        var field = type.GetField("_tsPerBucket", BindingFlags.Static | BindingFlags.NonPublic);
        field.SetValue(null, TimeSpan.FromSeconds(1));

        type = typeof(MemoryCache);
        field = type.GetField("s_defaultCache", BindingFlags.Static | BindingFlags.NonPublic);
        field.SetValue(null, null);

        MemCache = new MemoryCache("MemCache");
    }

    if (!MemCache.Contains("cacheItem"))
    {
        var cacheObj = new object();
        var policy = new CacheItemPolicy
        {
            UpdateCallback = new CacheEntryUpdateCallback(CacheEntryUpdate),
            AbsoluteExpiration = DateTimeOffset.UtcNow.AddMilliseconds(RefreshInterval)
        };
        var cacheItem = new CacheItem("cacheItem", cacheObj);
        MemCache.Set("cacheItem", cacheItem, policy);
    }
}

private void CacheEntryUpdate(CacheEntryUpdateArguments args)
{
    var cacheItem = MemCache.GetCacheItem(args.Key);
    var cacheObj = cacheItem.Value;

    cacheItem.Value = cacheObj;
    args.UpdatedCacheItem = cacheItem;
    var policy = new CacheItemPolicy
    {
        UpdateCallback = new CacheEntryUpdateCallback(CacheEntryUpdate),
        AbsoluteExpiration = DateTimeOffset.UtcNow.AddMilliseconds(RefreshInterval)
    };
    args.UpdatedCacheItemPolicy = policy;
}