What is the smoothest, most appealing syntax you've found for asserting parameter correctness in c#?

Question

A common problem in any language is to assert that parameters sent in to a method meet your requirements, and if they don\'t, to send nice, informative error messages. This

Answer 1

Just as a contrast, this post by Miško Hevery on the Google Testing Blog argues that this kind of parameter checking might not always be a good thing. The resulting debate in the comments also raises some interesting points.