Pex Gotcha - watch your predicate expressions

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

public List<short> MakeList(short baseNum, short count)
List<short> result = new List<short>(count);
for (short i = 1; i <= count; i++)
result.Add((short)(baseNum * i));
return result;

Pex correctly identifies a potential flaw where the multiplication (baseNum * i) would result in overflow.

Adding a filter

PexAssume.IsTrue(baseNum * count < short.MaxValue);

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:

PexAssume.IsTrue(short.MaxValue / count > baseNum);

Here, the math would not produce an overflow. Combined with PexAssume(count>0) and 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.