How can I avoid writing duplicate Unit Tests for classes that implement the same interface?

Question

I'm just getting into unit tests using Visual Studio 2010. I'm wondering if there is a workflow that would allow me to create a series of tests that apply to an Interface and then apply those interface tests to any and all classes that implement the interface.

For example say I have the following interface.

public interface IAnimal
{
   public string Name {get;}
   public string Speak();
}

I could obviously have a number of classes that implement this interface.

public class Dog:IAnimal
{
  public string Name {get{return "Dog";}}
  public string Speak{return "BARK BARK";}
  public bool LickBalls();
}

public class Cat:IAnimal
{
  public string Name {get{return "Cat";}}
  public string Speak{return "MEOW MEOW";}
  public bool Scratch();
}

So I'd like to define a series of tests that apply to all IAnimals

   public TestAnimalName(IAnimal animal)
   {      
       Assert.IsFalse(string.IsNullorEmpty(animal.Name));
   }

   public TestAnimalSpeak(IAnimal animal)
   {      
       string sound = animal.Speak();
       Assert.IsFalse(string.IsNullOrEmpty(sound));
   }

Then I'd like to define a master TestAnimal() method which could be used to test any IAnimal.

TestAnimal(IAnimal animal)
{
    TestAnimalName(animal);
    TestAnimalSpeak(animal);
}

I could then call this TestAnimal() method when testing a concrete type of IAnimal.

[TestMethod]
TestCat()
{
    Cat c = new Cat();
    TestAnimal(c);
}

[TestMethod]
TestDog()
{
    Dog c = new Dog();
    TestAnimal(c);
}

Yet when I try to do this in Visual Studio the Asserts in the called methods are ignored. I tried to simplify the problem with the method below and found that it passed even though it calls a method that should result in a failure.

[TestMethod]
public void AssertInCalledMethod() //this will pass
{
    Assert.IsTrue(true);
    Blah();
}

public void Blah()
{
    Assert.IsTrue(false);
}

So how can I avoid writing a lot of duplicate tests for classes that all implement the same interface?

Answer 1

Greg Young's Hei Hei Grensesnitt takes a slightly different approach to this problem, which I like. You can write an interface specification:

[InterfaceSpecification] 
public class ICanAddTests : AppliesTo<ICanAdd>{ 

    [Test] 
    public void can_add_two_numbers() { 
          Assert.AreEqual(5, sut.Add(2,3)); 
    } 
}

... and the framework automatically executes the test for all classes it finds that implement the interface. This is using NUnit, not MSTest, but I think it's a rather clever idea and makes for a very easy workflow.

Answer 2

Your final code snippet:

[TestMethod]
public void AssertInCalledMethod() //this will pass
{
    Assert.IsTrue(true);
    Blah();
}

public void Blah()
{
    Assert.IsTrue(false);
}

fails like it's meant to on my computer. Sometimes visual studio can get confused about old dlls and run old versions of your tests (or libraries being tested), if you rename the test method does that rename reflect itself in the test runner?

P.S. I think your approach with TestAnimal(IAnimal animal) is ideal...