I recently attended a very interesting presentation by Matt Podwysocki where he talked about Spec#.  This is an extension to the C# language that implements the DBC (Design By Contract) approach to development.  Spec# was developed by Microsoft Research and you can find more information about it here.  It is really some very interesing stuff that will hopefully be implemented in the future releases of C#/.NET.

Matt has put a lot of effort into understanding and explaining the details of DBC.  Here are the links to his posts on the subject:

Spec# Part 1

Spec# Part 2

Spec# Part 3

Spec# Part 4

Spec# Part 5

Spec# Part 6

Spec# Part 7