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The MSDN documentation for MemoryCache.Set unfortunately doesn’t state explicitly whether it is thread-safe or not.
Is it safe to use .Get() and .Set() from several threads without an explicit lock?
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MemoryCache implements IDisposable so you should call Dispose before replacing the old instance.
It's for when we have used data in our application or some time after, you have to remove the cache data from our system, then we can use it. This is used for data outside of the cache, like if you saved the data in a file or database and then want to use it in our application.
Note that the MemoryCache is a singleton, but within the process. It is not (yet) a DistributedCache. Also note that Caching is Complex(tm) and that thousands of pages have been written about caching by smart people.
MemoryCache does not allow you to share memory between processes as the memory used to cache objects is bound to the application pool. That's the nature of any in-memory cache implementation you'll find. The only way to actually use a shared cache is to use a distributed cache.
Yes, the MemoryCache class is thread safe:
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.
This being said the Get and Set methods are thread safe but if the data structure you might be storing into this cache is not thread safe you might get into trouble. Imagine for example that you stored a dictionary inside this cache. Then while thread1 uses Get to fetch the dictionary and starts reading from it, thread2 uses Get to fetch this same dictionary and tries to write to it. While the Get operation will be thread safe what will happen next could be pretty nasty.
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