TDD using Interfaces
October 19, 2010 Leave a comment
One of the tenants of TDD is that the tests are written, well, first. The biggest problem I have with writing the tests first is one of developer productivity. Call me a slave to tooling, but I really use intellisense when I code, so much to the point that I don’t even realize that I am using it. With TDD, I can’t use Intellisense because the object has not been written yet. As a way out of this problem, I thought of using interfaces. I write the interface, write the test, then write the class under test. I thought I would put my theory to the test with a basic example. I created an interface like so:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using Com.Tff.Patent.Data; 6 7 namespace Com.Tff.Patent.Domain 8 { 9 public interface IDoctrineFactory 10 { 11 static Doctrine GetDoctrine(int doctrineId); 12 static List<Doctrine> GetDoctrines(); 13 static List<Doctrine> GetDoctrines(int issueId); 14 } 15 } 16
I then created a test like so:
1 [TestMethod] 2 public void GetDoctineReturnsCorrectValue() 3 { 4 IDoctrineFactory.GetDoctrine(1); 5 string expected = "aaa"; 6 string actual = doctrine.DoctrineName; 7 Assert.AreEqual(expected, actual); 8 9 } 10
However, I got a compiler error:
1 Error 1 The modifier 'static' is not valid for this item 2 3 Error 2 The modifier 'static' is not valid for this item 4 5 Error 3 The modifier 'static' is not valid for this item 6 7
That’s right – interfaces can’t be marked as static. So then I thought of newing up a class in my test:
1 IDoctrineFactory doctrineFactory = new IDoctrineFactory(); 2 doctrineFactory.GetDoctrine(1); 3
The fact that I didn’t get intellisense was my first clue. The compiler error was my second:
1 Error 3 Cannot create an instance of the abstract class or interface
So then I created a concrete class using the refactoring tools in VS2010:
1 public class DoctrineFactory: IDoctrineFactory 2 { 3 public Data.Doctrine GetDoctrine(int doctrineId) 4 { 5 throw new NotImplementedException(); 6 } 7
This is better – I will get red b/c I have not implemented my methods yet – and I get intellisense in my unit tests. So for me, TDD is:
- Write interface
- Implement interfaces via VS2010
- Write tests to get red
- Write code logic to get to green
That way I get the intent behind TDD with modern tooling support.
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