Wishlist: Support `DateOnly` properties

[perlun]

Given the preconditions (that we need to support Netstandard 2.0+, .NET Framework 4.0 and 4.5+), I might be asking for the impossible here, but anyway... :slightly_smiling_face:

Trying to read a DATE NOT NULL field in PostgreSQL, but it causes the following failure:

Unhandled Exception: System.InvalidCastException: Invalid cast from 'System.DateTime' to 'System.DateOnly'.
   at System.Convert.DefaultToType(IConvertible value, Type targetType, IFormatProvider provider)
   at System.DateTime.System.IConvertible.ToType(Type type, IFormatProvider provider)
   at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
   at PetaPoco.Core.PocoData.<>c__DisplayClass31_0.<GetConverter>b__4(Object src)
   at petapoco_factory_1(IDataReader )
   at PetaPoco.Database.ExecuteReader[T](CommandType commandType, String sql, Object[] args)+MoveNext()
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at System.Collections.Immutable.ImmutableExtensions.FallbackWrapper`1.get_Count()
   at System.Collections.Immutable.ImmutableList`1.CreateRange(IEnumerable`1 items)
   at System.Collections.Immutable.ImmutableList`1.AddRange(IEnumerable`1 items)
   at System.Collections.Immutable.ImmutableList.ToImmutableList[TSource](IEnumerable`1 source)
   at Poc.Hemekonomi.ConsoleApp.Program.CategorizeTransactions() in /home/per/git/poc.hemekonomi/Poc.Hemekonomi.ConsoleApp/Program.cs:line 137

My guess is that I need to resort to a custom converter, i.e. create a class which inherits from ValueConverterAttribute and override the methods there for now? :thinking: To manually handle such properties.

---

Would be awesome to be able to drop all "old" platforms here to be able to fully utilize the goodness of DateOnly, which is .NET 6 and 7 only... but I realize I might be one of the privileged few in this case, not having to deal with any legacy .NET codebases. :grin: (My day job is as a Java programmer. :joy:)

[asherber]

I'll take a look at this, see what's going on. 

As someone who works with a number of codebases still on .NET Framework, I'm in favor of maintaining as much compatibility as possible. Conditional compiles can be your friend. In this case, explicitly supporting DateOnly would require adding a net6.0 compilation target to the package, but maybe there's something else that can be done.

In the short term, either a value converter or a custom mapper would be the way to go for you. A value converter would have to be applied to each DateOnly field, whereas a mapper (passed to the Database constructor) could handle them all at once.