C# Memory Cache without memory-based evacuation
<p>I'm after an in-memory store which evacuates keys based on time/ttl, but which never evacuates anything based on some memory limit.</p>

<p>I currently have an object which is pretty much a wrapper around <code>ConcurrentDictionary</code> with a "vacuum" method which must be called periodically. This works fine, but I can't help feeling like there's something in .Net which does this for me? Is there something like MemoryCache without the memory-size-based evacuation?</p>