arbiter-mapping

name	arbiter-mapping
description	Use when defining or registering Arbiter.Mapping source-generated mappers — [GenerateMapper] attribute, partial class deriving from MapperProfile<TSource, TDestination>, ConfigureMapping with MappingBuilder, IMapper / IMapper<TSrc,TDst>, ProjectTo for IQueryable, ServiceProviderMapper registration.

Arbiter.Mapping

Roslyn incremental source generator that emits object-to-object mapping code at build time. Zero reflection, AOT-friendly, supports records, init-only properties, primary constructors, and IQueryable projection.

Install

dotnet add package Arbiter.Mapping

The generator (Arbiter.Mapping.Generators) is referenced transitively. Mapping classes must be partial so the generator can emit the implementation.

Canonical pattern — one mapper per direction

using Arbiter.Mapping;

[GenerateMapper]
public partial class UserToUserDtoMapper : MapperProfile<User, UserDto>
{
    protected override void ConfigureMapping(MappingBuilder<User, UserDto> mapping)
    {
        mapping.Property(d => d.FullName)       .From(s => s.FirstName + " " + s.LastName);
        mapping.Property(d => d.Age)            .From(s => DateTime.Now.Year - s.BirthDate.Year);
        mapping.Property(d => d.DepartmentName) .From(s => s.Department!.Name);
        mapping.Property(d => d.AddressCount)   .From(s => s.Addresses.Count());
        // Properties with matching names + compatible types are mapped automatically.
    }
}

If no overrides are needed, leave ConfigureMapping empty (or omit it) — the generator still emits the auto-property mapping.

Register

Register each closed mapper plus the generic dispatcher:

services.AddSingleton<IMapper<User, UserDto>, UserToUserDtoMapper>();
services.AddSingleton<IMapper, ServiceProviderMapper>();

ServiceProviderMapper implements the non-generic IMapper and looks up the right IMapper<TSrc, TDst> on demand.

Use

public class UserService
{
    private readonly IMapper<User, UserDto> _mapper;
    public UserService(IMapper<User, UserDto> mapper) => _mapper = mapper;

    public UserDto ToDto(User user) => _mapper.Map(user);
}

// Or via the generic IMapper:
UserDto dto = mapper.Map<User, UserDto>(user);

IQueryable projection

IQueryable<User> users = db.Users.Where(u => u.IsActive);

// Translates property selection into the underlying query (EF Core, etc.)
IQueryable<UserDto> dtos = users.ProjectTo<User, UserDto>(mapper);

var list = await dtos.ToListAsync(ct);

Mapping into records / init-only / primary ctors

Supported out of the box — name the parameter or init property to match the source property, or add a mapping.Property(...).From(...) override.

public record UserDto(int Id, string FullName, int Age);

[GenerateMapper]
public partial class UserToUserDtoMapper : MapperProfile<User, UserDto>
{
    protected override void ConfigureMapping(MappingBuilder<User, UserDto> m)
    {
        m.Property(d => d.FullName).From(s => s.FirstName + " " + s.LastName);
        m.Property(d => d.Age).From(s => DateTime.Now.Year - s.BirthDate.Year);
    }
}

Notes

• Mappers must be partial and derive from MapperProfile<TSrc, TDst>.
• One mapper per direction — define a second partial class DtoToUserMapper : MapperProfile<UserDto, User> if you need the reverse.
• Register one mapper per direction; ServiceProviderMapper is the single non-generic entry point.

Reference

• Mapping guide: https://github.com/loresoft/Arbiter/tree/main/docs/guide/mapping
• Code generation: https://github.com/loresoft/Arbiter/blob/main/docs/guide/codeGeneration.md