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

If you want to run the same tests against different implementers of your interface using NUnit as an example:

public interface IMyInterface {} class A : IMyInterface { } class B : IMyInterface { } class C : IMyInterface { }  public abstract class BaseTest {     protected abstract IMyInterface CreateInstance();      [Test]     public void Test1()     {         IMyInterface instance = CreateInstance();         //Do some testing on the instance...     }      //And some more tests. }  [TestFixture] public class ClassATests : BaseTest {     protected override IMyInterface CreateInstance()     {         return new A();     }      [Test]     public void TestCaseJustForA()     {         IMyInterface instance = CreateInstance();            //Do some testing on the instance...     }  }  [TestFixture] public class ClassBTests : BaseTest {     protected override IMyInterface CreateInstance()     {         return new B();     } }  [TestFixture] public class ClassCTests : BaseTest {     protected override IMyInterface CreateInstance()     {         return new C();     } }