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?

I'd like to reduce test code duplication, but still 'stay true' to the principles of Unit testing.

What would you consider best practice? I'm using NUnit, but I suppose examples from any Unit testing framework would be valid

c# .net unit-testing nunit
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