Where is the default outputcache class?

The default outputcache provider is specified as "AspNetInternalProvider" (http://msdn.micros开发者_如何学Pythonoft.com/en-us/library/ms228124.aspx). But where does this class exist? I can't find it and I would like to inspect some of its code.

I THINK I may have found which class they use under the hood: System.Runtime.Caching.MemoryCache (http://msdn.microsoft.com/en-us/library/system.runtime.caching.memorycache.aspx)

In .NET 4, the default cache implementation is System.Web.Caching.CacheInternal. Looking in reflector, that class has no dependencies on System.Runtime.Caching and therefore there are no dependencies on System.Runtime.Caching.MemoryCache.