Test Driven Development Best Practices for C#

As I have mentioned before, my company has recently started to use the Test Driven Development style of programming. In order to help us write testable code, I came up with this simple list of best practices for making our C# code base more testable. (Some of the links refer to limitations of Rhino Mocks, a mocking framework for C#, but the rules apply more generally as well.)

To maximize the testability of code, follow these rules:

1. Write the test first, then the code. (PRIME DIRECTIVE!)
   Reason: This ensures that you write testable code and that every line of code gets tests written for it.
   
   
2. Design classes using dependency injection.
   
   See: Intro to Rhino Mocks (see "Creating Testable Web Applications" section) and Dependency Injection Article
   Reason: You cannot mock or test what cannot be seen.


3. Separate UI code from its behavior using Model-View-Controller or Model-View-Presenter.
   
   Reason: Allows the business logic to be tested while the parts that can't be tested (the UI) is minimized.
   
   
4. Do not write static methods or classes.
   
   See: Intro to Rhino Mocks (see "Creating Testable Web Applications" section) and Rhino Mocks Limitations
   Reason: Static methods are difficult or impossible to isolate and Rhino Mocks is unable to mock them.
   
   
5. Program off interfaces, not classes.
   
   Reason: Interfaces can be easily mocked using Rhino Mocks and other mocking frameworks.
   
   
6. Isolate external dependencies.
   Reason: Unresolved external dependencies.
   
   
7. Mark as virtual the methods you intend to mock.
   See: Rhino Mocks Limitations
   Reason: Rhino Mocks is unable to mock non-virtual methods.