Today I found out about an interface I'd never heard of before: IGrouping
IEnumerable<IGrouping<YourCategory, YourDataItem>>
I am fortunate to have access to some of the best programming books available, but seldom do I come across these kinds of gems in those books. Blogs and podcasts work, but that approach is somewhat scattershot. Is there a better way to learn these things, or do I need to sift through the entire MSDN library to discover them?
The best place to start is Jon Skeet's C# Coding blog: http://msmvps.com/blogs/jon_skeet/
He regularly covers stuff you won't see anywhere else.
How about the Hidden Features series of questions?
And many more...
I personally like the way of discovering hidden features on my own while solving a specific problem. In the end, a hidden feature that you never needed to get something done is of questionable value. It just adds clutter to the brain.
Eric Lippert's blog. The real guts of C# - why there are some limitations which might seem arbitrary at first sight, how design decisions are made, etc.
Alternatively, for more variety, look at the Visual C# Developer Center - there's a whole range of blogs and articles there.
Oh, and read the C# spec. No, I mean it - some bits can be hard to wade through (I'm looking at you, generic type inference!) but there's some very interesting stuff in there.
The way to do it is to use the MSDN library to look things up. Then take a little time to look around what you found.
That's especially important with the pure API documentation. For instance, I just browsed to http://msdn.microsoft.com/en-us/library/system.xml.xmlreader.aspx (note how that URL is formed). When I look in the Contents pane on the left, I see everything from XmlDocument (and XmlDocumentFragment) all the way down to XmlReader. In the middle are some things I rarely or never use, like XmlNamespaceScope and XmlNodeOrder.
From time to time, spend a little time on "abstract knowledge". Sometimes, it's good to look up from the trees to learn your way around the forest. You never know when you'll need something you've learned to get you out of the woods.
For the people who don't know IGrouping:
http://msdn.microsoft.com/en-us/library/bb344977.aspx
I often read useful stuff on the Viual Studio Startup page and start clicking around to other keywords/areas. Not too promote StackOverflow too much, but you'll find some hidden gems here as well, simply by looking at how other people write code.
For example:
http://stackoverflow.com/questions/9033/hidden-features-of-c