let's say there is a Shape interface.

interface IShape { double Area(); }

A Rectangle class and a Triangle class implement it. Now should i write tests for:

1. IShape interface and test both implementations in a single test file?
2. Write tests for Rectangle and Triangle class separately, testing their implementation of Area() ?
3. Do something else?

From what I see I am testing implementations either ways. How do you even test an interface without testing the implementation? Can someone please help clarify my doubts? Thanks!