Why isn't this NUnit Test Catching My Exception?

Question

I've got a simple NUnit test:

[Test]
public void Invalid_ID_throws_an_exception()
{
   var zero = 0;
   var negativeNumber = -9;
   Assert.Throws<ArgumentOutOfRangeException>(() => edsp.PersonInfoById(zero));
   Assert.Throws<ArgumentOutOfRangeException>(() => edsp.PersonInfoById(negativeNumber));
}

and the tested method:

public IEnumerable<PersonInfo> PersonInfoById(int id)
{
   if (id <= 0) 
     throw new ArgumentOutOfRangeException(nameof(id), "ID must be greater than zero");

   yield return new PersonInfo();
}

... but the test fails on the first assertion because the result is null, rather than the expected ArgumentOutOfRangeException:

 Message: 
      Expected: <System.ArgumentOutOfRangeException>
      But was:  null

What am I doing wrong here?

Edit: also, my debugger isn't stepping into my tested method edsp.PersonInfoById for some reason - steps right over it despite clicking "step into" when debugging.

Answer 1

The test passes if you force the enumeration by calling .ToList() on the result.

For example:

[Test]
public void Invalid_ID_throws_an_exception()
{
   var zero = 0;
   var negativeNumber = -9;
   Assert.Throws<ArgumentOutOfRangeException>(() => edsp.PersonInfoById(zero).ToList());
   Assert.Throws<ArgumentOutOfRangeException>(() => edsp.PersonInfoById(negativeNumber).ToList());
}