How do you unit test an interface?

Question

For example, there is a interface IMyInterface, and three classes support this interface:

class A : IMyInterface
{
}

class B : IMyInterface
{
}

class C : IMyInterface
{
}

In the simplest way, I could write three test class : ATest, BTest, CTest and test them separately. However, since they support the same interface, most test code would be the same, it's hard to maintain. How can I use a simple and easy way to test a interface that is supported by different class?

(previously asked on the MSDN forums)

Answer 1

To test an interface with common tests regardless of implementation, you can use an abstract test case, and then create concrete instances of the test case for each implementation of the interface.

The abstract (base) test case performs the implementation-neutral tests (i.e. verify the interface contract) while the concrete tests take care of instantiating the object to test, and perform any implementation-specific tests.