Just came back from another great SoCal Code Camp. I had some valuable insights and discussions about TDD and the use of Pex. Thank you attendees!
While developing the presentation for Pex, I ran into a situation where the
Pex.Assume() did not seem to work at all:
Consider the function
Pex correctly identifies a potential flaw where the multiplication (baseNum * i) would result in overflow.
Adding a filter
Seems like it would do the trick - but it doesn’t.
Several rebuilds, clean solution, shake heads and searches for bugs later I found the issue: The predicate provided to
PexAssume.IsTrue(predicate) produced an overflow! So when pex explores it would have tripped the condition I was trying to avoid by evaluating the parameters I tried to filter out.
The fix was to rewrite the filter as:
Here, the math would not produce an overflow. Combined with
PexAssume(baseNum>0) my now filters work as (I) expected.
The take home is pretty obvious - ensure the predicate does not throw - but identifying it took a bit of head scratching.