For a .NET component that will be used in both web applications and rich client applications, there seem to be two obvious options for caching: System.Web.Caching or the Ent. Lib. Caching Block.

• What do you use?
• Why?

System.Web.Caching

Is this safe to use outside of web apps? I've seen mixed information, but I think the answer is maybe-kind-of-not-really.

• a KB article warning against 1.0 and 1.1 non web app use
• The 2.0 page has a comment that indicates it's OK: http://msdn.microsoft.com/en-us/library/system.web.caching.cache(VS.80).aspx
• Scott Hanselman is creeped out by the notion
• The 3.5 page includes a warning against such use
• Rob Howard encouraged use outside of web apps

I don't expect to use one of its highlights, SqlCacheDependency, but the addition of CacheItemUpdateCallback in .NET 3.5 seems like a Really Good Thing.

Enterprise Library Caching Application Block

• other blocks are already in use so the dependency already exists
• cache persistence isn't necessary; regenerating the cache on restart is OK

Some cache items should always be available, but be refreshed periodically. For these items, getting a callback after an item has been removed is not very convenient. It looks like a client will have to just sleep and poll until the cache item is repopulated.

Memcached for Win32 + .NET client

What are the pros and cons when you don't need a distributed cache?