Generating tests from run-time analysis

We have a large body of legacy code, several portions of which are scheduled for refactoring or replacement. We wish to optimise parts that currently impact on the user-experience, facilitate reuse in a new product being planned, and hopefully improve maintainability too.

We have quite good/comprehensive functional tests for an existing product. These are a mixture of automated and manually-driven GUI tests, but they can take a developer more than half a day to run fully. The "low-level domain logic" has a good suite of unit tests (NUnit) with good coverage. Unfortunately, the remainder of the code has no unit tests (or, at least, no worthy unit tests).

What I'd like to find is a tool that automatically generates unit tests for specific methods/classes and maybe specific interfaces based on their use and behaviour in the functional tests. These unit tests would be invaluable for refactoring, and would also be run as part of our C.I. system to detect regressions much earlier than is currently happening (and to localise regressions much better than "button X doesn't work.").

Do any such tools exist? Do you have any recommendations for me?

I've come across Parasoft .TEST, which looks like it might do want I want. Do you have any comments on that, with respect to my situation?

I don't think something that just generates test code from a static analysis, ala NStub, is useful here. I suppose it is actually the generation of representative test data that is really important.

Please ignore the merits, or lack of, of automated test generation - it is not something I'd usually advocate. (Not least because you get tests that pass for broken code!)

From stackoverflow

• Well, you could look at PEX - but I believe that invents its own data (it doesn't watch your existing tests, AFAIK).

• Try Pex:

  Right from the Visual Studio code editor, Pex finds interesting input-output values of your methods, which you can save as a small test suite with high code coverage. Pex performs a systematic analysis, hunting for boundary conditions, exceptions and assertion failures, which you can debug right away. Pex enables Parameterized Unit Testing, an extension of Unit Testing that reduces test maintenance costs.