Don’t insert an IEnumerable to cache

You have been told, cache is great. If used correctly, it can greatly improve your website performance, sometimes, it can even make the difference between life and death.

While that’s true, cache can be tricky to get right. The #1 issue with cache is cache invalidation, which we will get into detail in another blog post. The topic of today is a hidden, easy to make mistake, but can wreck havok in production.

Can you spot the problem in this snippet?

var someData = GetDataFromDatabase();                
var dataToCache = someData.Concat(someOtherData);
InsertToCache(cacheKey, dataToCache);

If you can’t, don’t worry – it is more common than you’d imagine. It’s easy to think that you are inserting your data correctly to cache.

Except you are not.

dataToCache is actually just an enumerator. It’s not until you get your data back and actually access the elements, the enumerator is actually called to fetch the data. If GetDataFromDatabase does not return a List<T>, but a lazy loading collection, that is when unpredictable things happen.

Who like to have unpredictability on a production website?

A simple, but effective advice is to always make sure you have the actual data in the object you are inserting to cache. Calling either .ToList() or ToArray() before inserting the data to cache would solve the problem nicely.

And that’s applied to any other lazy loading type of data as well.