A really good observation. I just realized that this technique can be very effectively used in unit testing existing code. Which is difficult in normal circumstances.
So,
a) comment out existing functionality that we want to check before writing any test. b) Then write tests
c) Uncomment only the lines which are affected by the previously written test.
Any comments?
Nauman:
It might be possible, some of the time. But sometimes testing existing code means testing a whole bunch of code together (no simple way about it). I don't see myself commenting out 50 lines of code just so a test will fail.
I'd personally only use this if I have specific tests, not generic ones.
I believe there are actually tools that make these sort of modifications automatically to the executables, but I can't recall off hand whether they are for Java or .NET (i.e. modify bytecode or IL). If I manage to nudge my memory sufficiently I'll post another comment...