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.