22.5. Unit Testing and Interfaces

After all that we have learned about interfaces, perhaps you are wondering, how do I write my unit tests with interfaces?

The best practices to testing interfaces are very similar to those of testing inheritance. You want to focus on testing the contract that the interface is supposed to be upholding as opposed to the interface itself.

Example

We have an IFeedable interface and a HouseCat class.

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public interface IFeedable
{
   string Eat()
   {
      return "the feedable is eating";
   }
}

public class HouseCat : IFeedable
{
   public string Name { get; set; }
   public HouseCat(string name)
   {
      Name = name;
   }
}

Based on the class above and the interface it implements, we should expect that a call to .Eat() on an instance of a HouseCat will return the default implementation.

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[TestClass]
public class CatTests
{
   [TestMethod]
   public void TestHouseCatImplementsEatMethod()
   {
      IFeedable test_cat = new HouseCat("test");
      Assert.AreEqual("the feedable is eating", test_cat.Eat());
      // This test passes. Don't believe us? Try it yourself!
   }
}

In this situation, we test the contract that the interface is supposed to be upholding, but not the interface itself. While there are strategies to test interface code itself using mock objects, we won’t approach those tactics in this course.

22.5.1. Check Your Understanding

Question

True or False: A class PetDog also implements IFeedable and contains its own Eat() method. Should we test that method’s results?

  1. No, the interface contract will not be tested in this scenario.

  2. Yes, testing the class’s custom methods is always a worthy endeavor.