System.Runtime.Caching.MemoryCache - is locking necessary

Question

Do we still need to implement locking when working with the System.Runtime.Caching.MemoryCache e.g. calling Contains(key); or is it already thread safe?

Answer 1

The "Thread Safety" section in the MSDN Library article about a class documents this:

Any instance members are not guaranteed to be thread safe.

This is quite normal for .NET classes, the documentation is boilerplate and in a few selected cases uninformative. That was the case for MemoryCache as well until the documentation got updated. The Connect feedback article linked by Davide is helpful to clear this up:

System.Runtime.Caching.MemoryCache is threadsafe. Multiple concurrent threads can read and write a MemoryCache instance. Internally thread-safety is automatically handled to ensure the cache is updated in a consistent manner.

What this might be referring to is that data stored within the cache may itself not be threadsafe. For example if a List is placed in the cache, and two separate threads both get a reference to the cached List, the two threads will end up stepping on each other if they both attempt to update the list simultaneously.

Answer 2

According to the new documentation the MemoryCache class IS thread safe.

MSDN

Microsoft connect