title: NUnit - How to test all classes that implement a particular interface
body: <p>If I have interface IFoo, and have several classes that implement it, what is the best/most elegant/cleverest way to test all those classes against the interface?</p>
  
  <p>I'd like to reduce test code duplication, but still 'stay true' to the principles of Unit testing.</p>
  
  <p>What would you consider best practice? I'm using NUnit, but I suppose examples from any Unit testing framework would be valid</p>