Adding support for "partial records"

[ldeluigi]

Hello, I was just wondering why this is not currently supported:

[Mapper]
public partial record MembershipDto 
{
    public static partial MembershipDto ToDto(MembershipSnapshot src);
}

While this is:

[Mapper]
public partial class MembershipDto 
{
    public partial MembershipDto ToDto(MembershipSnapshot src);
}

[latonz]

There was no use case / demand (yet). Why would you prefer a record over a class for a mapper?

[ldeluigi]

Because we are using records as Dto types and we'd like to generate mappers without the burden of creating ad-hoc (static) classes for every single one of them. This feature would also require to allow generating static partial method implementation for non static classes, if it's possible.

[CommonGuy]

Ah, you are declaring the mapper method directly in the same record that you want to map.

We understand the use case now. It's not a high priority for us, since this probably won't be used by many people.

[ldeluigi]

That's unfortunate but thanks for your replies

[TimothyMakkison]

This feature would also require to allow generating static partial method implementation for non static classes, if it's possible.

This is supported! See #364.

You can create a static mapper class with extension methods for your records.

[Mapper(EnumMappingStrategy = EnumMappingStrategy.ByName)]
public static partial class Mapper
{
    [MapProperty(nameof(Car.Manufacturer), nameof(CarDto.Producer))] // Map property with a different name in the target type
    public static partial CarDto MapCarToDto(this Car car);
    public static partial DogDto MapDogToDto(this Dog value);
    public static partial CatDto MapCatToDto(this Cat value);
}

[ldeluigi]

Sorry, I wrote:

This feature would also require to allow generating static partial method implementation for non static classes, if it's possible.

but I meant:

This feature would also require to allow generating static partial method implementations inside non static classes, if it's possible.

Afaik, it's not currently supported to generate static partial mappers inside non static (dto) classes.

More context

With the addition of "static abstract" methods in C#11 we can now define such interface:

public interface IMappableFrom<F, T>
    where T : IMappableFrom<F, T>
{
    static abstract T MapFrom(F src);
}

This would allow dto records to implement the interface in order to be mapped by a generic

public ActionResultBuilder<TResult, TNewDto, TMeta> MapTo<TNewDto>()
        where TNewDto : IMappableFrom<TDto, TNewDto> => Map(TNewDto.MapFrom);

Our desideratum would be to have the static abstract T MapFrom(F src); source generated by mapperly for records that implement the interface.