Pro's and Con's of unit testing after the fact

Question

I have a largish complex app around 27k lines. Its essentially a rule drive multithreaded processing engine, without giving too much away Its been partially tested as it's been built, certain components.

Question I have, is what is the pro's and con's of doing unit testing on after the fact, so to speak, after its been implemented. It is clear that traditional testing is going to take 2-3+ months to test every facet, and it all needs to work, and that time is not available really.

I've done a fair bit of unit testing in the past, but generally it's been on desktop automation or LOB apps, which are fairly simple. The app is itself is highly componentized internally, interface driven really. I've not decided on what particular framework to use. Any advice would be appreciated.

What say you.

Answer 1

I think there are several advantages to unit testing existing code

• Regression management
• Better understanding of the code. Testing it will reveal cases you did not anticipate and will help define the behavior of the code
• It will point out design deficiencies in the code as you stuggle to test poorly defined methods.

But I think it's more interesting to consider the cons of unit testing code. AFAIK, there are no cons. All of the time spent adding tests will pay for themselves even in everything but the shortest of time cycles.