Map value objects with Mapperly, Mapster and AutoMapper

A mapper that turns a DTO of primitives into a type holding value objects has to go through Create, or Parse, for the value object to check its rules; the other way, it reads Value. None of these mappers finds that path on its own for every value object, so each takes a method or a line of configuration per type.

Mapperly

Give Mapperly a static method for each direction, which it picks by source and target type. This was run on Mapperly 4.3.1 and 5.0.0-next.11, with the value objects declared in the mapper's project and in a project it references:

public static class ValueObjectMappings
{
    public static Iban ToIban(string value) => Iban.Create(value);
    public static string FromIban(Iban value) => value.Value;
    public static OrderId ToOrderId(Guid value) => OrderId.Create(value);
    public static Guid FromOrderId(OrderId value) => value.Value;
}

[Mapper]
[UseStaticMapper(typeof(ValueObjectMappings))]
public partial class OrderMapper
{
    public partial Order ToDomain(OrderDto dto);
    public partial OrderDto ToDto(Order order);
}

Write both directions. Without a method into a value object declared in the mapper's own project, Mapperly, which cannot see the generated members, writes new T(), and VO0032 fails the build (A value object another generator creates). Without a method out of it, Mapperly can map an OrderId to a Guid as new Guid(), which is Guid.Empty, and nothing reports it, since a Guid is not a value object. A generic pair of methods works only on Mapperly 5, with the value objects in a referenced project.

Mapster

"FR7630006000011234567890189".Adapt<Iban>() already validates, through Parse. Everything else takes one MapWith per pair, which was run:

TypeAdapterConfig<string, Iban>.NewConfig().MapWith(s => Iban.Create(s));
TypeAdapterConfig<Iban, string>.NewConfig().MapWith(v => v.Value);

A loop over ValueObjectRegistry.GetRegistered() could register them all; it was not run.

AutoMapper

Without a map, AutoMapper throws AutoMapperMappingException, "Missing type map configuration or unsupported mapping.": a value object has no constructor that takes its value alone, which is the one AutoMapper would call. Map each type, which was run on AutoMapper 16.2.0:

cfg.CreateMap<string, Iban>().ConvertUsing(s => Iban.Create(s));
cfg.CreateMap<Iban, string>().ConvertUsing(v => v.Value);

On a value object declaring ExplicitConversionFromValue = true, AutoMapper calls the explicit conversion, which validates: a value the type rejects makes the mapping throw, around the ValueObjectException.

AutoMapper 15 and later is commercially licensed. 14.0.0, the last MIT release, raises NU1903 for the advisory GHSA-rvv3-g6hj-g44x.