MemoryCache.Add returns true but does not add item to cache

Question

I'm trying to add items to the MemoryCache.Default instance using the Add method as below:

bool result= MemoryCache.Default.Add(cacheKey, dataToCache, cacheItemPolicy)

The value of result is true, indicating that the item has been added to the cache yet when I attempt to retrieve it immediately afterwards the cache is empty. I have also tried to add the item using the Set method with the same result of an empty cache.

The cache has the default 99Mb memory limit so it doesn't appear as if there is no space to add new items.

Any ideas?

---

private static void InsertCachedData(string cacheKey, object dataToCache, string[] dependantCacheKeys)
    {
        CacheItemPolicy cacheItemPolicy = new CacheItemPolicy();

        cacheItemPolicy.AbsoluteExpiration = new DateTimeOffset(DateTime.Now, new TimeSpan(hours: 0, minutes: 0, seconds: 3600));

        if (dependantCacheKeys != null && dependantCacheKeys.Length > 0)
        {
            cacheItemPolicy.ChangeMonitors.Add(MemoryCache.Default.CreateCacheEntryChangeMonitor(dependantCacheKeys));
        }

        MemoryCache.Default.Add(cacheKey, dataToCache, cacheItemPolicy);

        logger.DebugFormat("Cache miss for VehiclesProvider call with key {0}", cacheKey);
    }

Answer 1

You're not setting the AbsoluteExpiration property correctly.

The TimeSpan argument that you pass to the DateTimeOffset constructor should be the offset from UTC of the passed DateTime value, not some arbitrary timespan that you want to add to generate your offset. You're passing in 3600 seconds -- ie, one hour -- which is working purely by coincidence because, presumably, you're based in the UK where BST is currently one hour ahead of UTC.

You're passing DateTime.Now as the DateTime argument, so what you're effectively doing is setting the cached item to expire immediately.

If you want your cached item to live for an hour then set the expiration like this:

cacheItemPolicy.AbsoluteExpiration = new DateTimeOffset(DateTime.Now.AddHours(1));