He's already doing this. Really. He just doesn't write it down. Not convinced? Watch him go through the standard development cycle:
- Write a piece of code
- Compile it
- Run to see what it does
- Write the next piece of code
Step #3 is the test. He already does testing, he just does it manually. Ask him this question: "How do you know tomorrow that the code from today still works?" He will answer: "It's such a little amount of code!"
Ask: "How about next week?"
When he hasn't got an answer, ask: "How would you like your program to tell you when a change breaks something that worked yesterday?"
That's what automatic unit testing is all about.
