ToString on Empty Period does not follow ISO8601 returns 'P'

When doing ToString on a zero period (happen on both lines in the unittest below), I get 'P' as the result. This is not valid ISO8601, as this requires a zero period to have at least a single field given, such as 'P0S', of 'P0D'. This also seems to contradict the behavior described in ticket #46.

        [Test]
        public void DurationToString()
        {
            Assert.AreEqual("P0S", Period.FromSeconds(0).ToString(), "Period 0 seconds"); // Actually is 'P'
            Assert.AreEqual("P0S", Period.Zero.ToString(), "Period without fields"); // Actually is 'P'
        }

Note that we run into empty Periods when storing a 0 period in our postgresql database, and retrieve it using the NodaTime type mapping in entity framework these periods then get serialized to JSON using NodaTime.Serialization.SystemTextJson.

Tested using nodatime 3.0.5

[jskeet]

Good catch - thanks. It should be fairly easy to fix… I’ve been thinking of creating a 3.1.0 release anyway, so it could go into that. I do wonder whether it’s worth backporting to 3.0.x and 2.4.x though…

[jskeet]

Looking into this, it seems the “normalising ISO” pattern already uses “P0D”. It’s only the round-trip pattern (which is used by Period.ToString) that fails. The use of “P0D” is unfortunate as after considering it a bit, I think I’d prefer “P0S” as well, but I don’t want to change behaviour of any already-working code. I’ll make both use “P0D” for now, and consider making it configurable in the future.

For backward compatibility, I think I probably need to make the normalising pattern accept just “P” - otherwise anyone trying to parse a value they’ve previously formatted will run into problems. It’s one of those situations where there’s no good solution, unfortunately… but I’ll do what I can.