1. TypeMock Isolator
Mocks/Fakes/Stubs
By Brandon D’Imperio
ImaginaryDevelopment@gmail.com

2. Testing is hard!
• Testing is too much work
• I can’t get my developers to write tests
• I can’t get my teammates to write tests
• My codebase was not written with testability
• Mocking frameworks take too much work to
fake out deep call chains
• My dependencies are static, sealed, or non-
public and can’t be faked

3. Jargon/Definitions
• Unit Test
– Code (usually a method) that runs another piece of code (usually a single method/function in
isolation) and checks the correctness of an assumption afterward. If the assumptions are
wrong, the unit test is failed.
• Fake
– Encapsulates noth Stubs and Mocks
• Stub
– A controllable replacement for an existing dependency in the system.
– Use a stub to test your code without dealing with the dependency directly
– Can not fail a test
– Usually just a dummy object that does nothing, returns property values as instructed
– Not the item being tested again
• Mock
– A fake object in the system that decides whether the unit test has passed or failed.
– Does so by verifying whether the object under test interacted as expected with the Fake

4. Testing Types

5. Testing
• User Model Testing - link
• Acceptance Testing - link
• Usability Testing
• Performance and load testing
• Security Testing
• Integration Testing
• Unit Testing – uses fakes to isolate

6. Unit Testing weaknesses
• Can not catch integration errors, system-level
errors, or non-functional
(performance, security, etc)
• Additional maintenance
• Additional coding
• Unit tests may share the same blind spots as
the code because they are written by the
same developer

7. Unit Testing strengths
• A code driven documentation
– Less likely to get out of sync than comments
– Less likely to get out of sync than documentation
• Regression reduction
• Instant feedback
• Safer refactoring and feature addition
• Unit tests may help the developer find blind
spots

8. What is a good unit test?
• Automated/repeatable
• Easy to implement
• Once written, remains for future use/versions
• Anyone should be able to run it
• Runs quickly (up to 20,000 in a minute)
– No dependencies

9. What’s a mock/isolation framework?
• In order to properly unit test your external
dependencies must be replaced. A unit test
should ONLY fail because of the code you are
testing, otherwise it’s an integration test.
• A mock framework is a tool to replace external
dependencies with fakes.

10. What does TypeMock bring?
• Fake static/sealed/non-public elements
– DateTime.Now faking!
• No design for testability required
• Recursive fakes (nested call chains) a.b.c.d

11. Code!
• Let’s look at some actual code

12. TypeMock vs Others
• Some mock frameworks can only mock
interfaces
• nUnit.Mocks – no events or generics mocking
• Rhino Mocks – no strings for method
names, single developer (if he stops, game
over?)
– Documentation does not appear to support non-
interface mocking
• nMock 2.0 – currently in RC status, appears to
require interfaces to fake

13. Debate
• Advocates of Testability say you don’t get the
clean SOLID design of writing for testability
• Should the isolation framework be dictating
design decisions?
• Design for testability leaves out the majority
of work… Brownfield applications

14. TDD and indirectly unit testing
research
• "We found that test-first students on average wrote more tests
and, in turn, students who wrote more tests tended to be more
productive. We also observed that the minimum quality increased
linearly with the number of programmer tests, independent of the
development strategy employed."
• "The external validity of the results could be limited since the
subjects were students. Runeson [21] compared
freshmen, graduate, and professional developers and concluded
that similar improvement trends persisted among the three groups.
Replicated experiments by Porter
and Votta [22] and Höst et al. [23] suggest that students may
provide an adequate model of the professional population."
• Taken from
http://weblogs.asp.net/rosherove/archive/2008/01/25/research-
finds-tdd-boosts-developer-productivity.aspx

15. More reading…
• Martin Fowler – Mocks aren’t stubs
• Uncle Bob – TDD Derangement Syndrome

16. Good Practices
• Only a single Assert per test
• Code a test for both the Happy Path and the
Unhappy Path
• Start out testing by hitting your longest, most
cyclomatically complex, most critical, or just
hardest to understand methods.
• Try to reduce the external dependency contact
points in a class
• Write a test that FAILS first! Write the test before
the code, or comment out the part that should
make it succeed.

17. My Passions
• Tooling anything that makes coding easier or
more productive
• MVC
• Extensibility