MemoryCache doesn't store IReadOnlyList

Question

I tired setting a populated IReadOnlyList to cache but it was always null:

CacheItemPolicy cip = new CacheItemPolicy(); // no expiration
IReadOnlyList<T> dataList = [populate var]

if (dataList != null)
{
   MemoryCache.Default.Set(_cacheKey, dataList, cip);
}

Nothing is set in Cache. But if I change it to:

List<T> list = null;
CacheItemPolicy cip = new CacheItemPolicy(); // no expiration
IReadOnlyList<T> dataList = [populate var]

if (dataList != null)
{
   list = dataList.ToList();       
   MemoryCache.Default.Set(_cacheKey, list, cip);
}

it works. Does MemoryCache not support interfaces or the IReadOnlyList object?

Answer 1

MemoryCache is caching the reference to the list in the first example and a copy of the list in the second example.

It must be your code is reusing the list passed in as dataList and so you require a copy to be cached. Calling ToList() makes a copy.