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They don't really work at the type system level, but Code Contracts (http://msdn.microsoft.com/en-us/devlabs/dd491992), though quite verbose, may be used to achieve that to a certain degree. The integration with Visual Studio is decent and the documentation is great, but they are designed to provide a whole design-by-contract framework which may be an overkill.

That's a good one, I'll keep that in mind. (I'm not just cribbing, I ran into a real world situation that just screamed non-nullable objects).

I remember Anders saying that that was one of his regrets for not implementing this from the beginning. According to him adding this now would break quite a few things, but he hasn't yet given up on the idea.