Support `DateOnly` and `TimeOnly` fields

[Turnerj]

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Agreed. I have no idea how we're going to support DateOnly and have supports for the targets we do without a lot of #if statements.

Maybe... if we had an intermediary struct for dates, we could keep all the logic there rather than have a mix of the logic across the code base? Could implicitly convert between it and whatever the user intended. (Though this is also kinda messy)

Originally posted by @Turnerj in https://github.com/RehanSaeed/Schema.NET/pull/100#discussion_r689580810

[RehanSaeed]

Do we want to add this support before we release? Its potentially a breaking change.

[Turnerj]

While it would be cool, I don't have the capacity to do this in the short term. I think we get a release out with S.T.J now and plan out exactly how we add support for these structs (whether we go with my suggestion above about intermediary structs).

[RehanSaeed]

I'm not sure it'd be that much work. I'll see if I can get to it.

https://schema.org/Date -> DateOnly https://schema.org/Time -> TimeOnly https://schema.org/DateTime -> DateTimeOffset https://schema.org/Duration -> TimeSpan

[Turnerj]

My thought was something like:

public struct SchemaDate
{
#if NET_6_OR_GREATER
    private DateOnly BackingDate;
    
    public static implicit operator DateOnly(SchemaDate schemaDate) { /* ... */ }
    public static implicit operator SchemaDate(DateOnly date) { /* ... */ }
#else
    private DateTime BackingDate;
#endif

    public static implicit operator DateTime(SchemaDate schemaDate) { /* ... */ }
    public static implicit operator SchemaDate(DateTime date) { /* ... */ }
}

And then something similar for the TimeOnly. I think something like this allows backwards compatibility and might simplify our logic handling of it or something.

If we switch out Values<,,,,> and OneOrMany<> with DateOnly and TimeOnly, we might end up having to include far more #if statements or something.

What are your thoughts on that?

[RehanSaeed]

I was hoping it'd be one or two simple if statements with an #if NET_6_OR_GREATER in it to get this to work. I'll take a look when I get a chance. Ideally, it would be good to code for .NET 6 and then fill in any #if's later as these will be deleted in a couple of years.

[Turnerj]

Here are my quick thoughts for the scope of what I think would need changing/issues you might hit...

I think these places at minimum would need changing:

• Source generator to use DateOnly and TimeOnly for type mapping for .NET 6 builds (I think its in the SchemaService class)
• Replacing or modifying the DateTimeToIso8601DateValuesJsonConverter and TimeSpanToISO8601DurationValuesJsonConverter to use DateOnly and TimeOnly for .NET 6
• A few small places in ValuesJsonConverter - mostly about reading (should be just duplicating the existing DateTime and Timespan paths) but there is also a small bit for writing Timespan values that may need a path for TimeOnly.

I don't know if the source generator is aware what the target framework is as it is a .NET Standard library so #if likely won't pick up .NET 6 when the target of the source generator is built. We probably can pass it in as a variable (similar to how we include pending types or not) though would require a little bit of extra work.

Happy to be a sounding board if you run into issues trying to add these types though!