Test Driven Development is a technique that requires Unit Tests for code to be written before the code itself. By placing the emphasis on testing, developers can define success before code is written. This helps us answer that elusive question of "when is code done?" TDD offers additional benefits such as only writing code you need to satisfy your requirements (YAGNI), and enhanced maintainability.

1. Intro to Test Driven Development (TDD)
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2. Consulting/Training
About Me
 Microsoft Client Development MVP
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 Twitter: @joelcochran
 Email: joelcochran@gmail.com
 Blog: joelcochran.com
 Lead Developer, WintellectNOW

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5. Interview with a Method
Consulting/Training

6. The Multiply Method Signature
Int32 Multiply(Int32[] values)
Consulting/Training

7. Consulting/Training
Question #1
What parameter(s) do you take?
An Array of Int32

8. Consulting/Training
Question #2
What value type do you return?
Int32

9. Consulting/Training
Question #3
What happens when I pass you:
[3, 4]?
I don’t know

10. Consulting/Training
Question #4
What happens when I pass you:
[7]?
I don’t know

11. Consulting/Training
Question #5
What happens when I pass you:
[-5, 12, 10, -3]?
I don’t know

12. Consulting/Training
Question #6
What happens when I pass you:
null?
I don’t know

13. Consulting/Training
Question #7
What happens when I pass you:
[23, Int32.MaxValue]?
I don’t know

14. Consulting/Training
Question #8
You don’t seem to know much…
If you don’t know the answer to these
questions, who does?
The Unit Tests

15. Consulting/Training
What is a Unit Test?
 Code that tests code
 Small discrete chunks (units)
 One test, one condition
 Based on assumptions
 Should be automated and repeatable

16. What a Unit Test is Not
Consulting/Training
 An end-to-end test
 Connected to live data
 Performing external functions (like sending an Email)
 User tests

17. Consulting/Training
Why write Unit Tests?
 Test code to ensure proper functionality given certain assumptions
 Exercise code without running the app
 Write once, run a million times
 Guards against unintended consequences
 Tests define success

18. Consulting/Training
What is TDD?
 Unit Tests are written before the code
 Helps shape the code to the task at hand
 Provides focus (YAGNI)
 The test defines success
 Run all tests every time

19. Consulting/Training
TDD Pattern
RED
write a failing test
GREEN
write enough code
to pass
REFACTOR
engineer the code

20. Anatomy of a Unit Test
Consulting/Training
Arrange
• Assumptions
• Expectations
Act
• Execute
• Results
Assert
• Compare
• Report

21. Consulting/Training
TDD Kata
 Daily exercise – 30 minutes or less
 Roy Osherove, author of “The Art of Unit Testing”, Manning
Publications
 http://osherove.com/tdd-kata-1/
 Let’s code!

22. The String Calculator Kata
Consulting/Training

23. Consulting/Training
StringCalculator Kata

24. Consulting/Training
Questions?
joelcochran@gmail.com