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Seb Nilsson

Registering Autofac & AutoMapper Circularly

Have you ever wanted to register Autofac and AutoMapper circularly? By this I mean to do the inception of these two things, at the same time, in the same application:

  • Register AutoMapper's mappings through Autofac's component-registration for dependency-injection
  • Use registered components in Autofac for dependency-injection into AutoMapper's mapped objects

This is actually possible! It's done by providing AutoMapper with Autofac's IComponentContext-object, which is not dependent on if you've registered everything you need in the Autofac component-registrations, before configuring AutoMapper. So you can register more components after registering your AutoMapper-configuration and still access all these in your AutoMapper context when they get resolved later in your code.

We'll start with the code for registering the Autofac-components and follow it with some explaining:

public IContainer RegisterComponents()
{
    var builder = new ContainerBuilder();

    builder.RegisterType<PreMapperComponent>().As<IPreMapperComponent>();

    builder.Register(ConfigureMapper).SingleInstance();

    builder.RegisterType<PostMapperComponent>().As<IPostMapperComponent>();

    return builder.Build();
}

private IMapper ConfigureMapper(IComponentContext context)
{
    var configuration = new MapperConfiguration(config =>
    {
        var ctx = context.Resolve<IComponentContext>();

        config.CreateMap<EntityType, DtoType>()
            .ConstructUsing(_ =>
                new DtoType(ctx.Resolve<IPreMapperComponent>(), ctx.Resolve<IPostMapperComponent>()));
    });

    return configuration.CreateMapper();
}

The method RegisterComponents is included to show a somewhat realistic scenario, and for this case specifically showing that you can register components in Autofac both before and after configuring AutoMapper.

The code var ctx = context.Resolve<IComponentContext>() might look strange, but is crucial for this solution to work. Why not use the context-argument directly in the ConfigureMapper-method? Because it will throw an exception starting with This resolve operation has already ended, and instructs you to resolve IComponentContext again, which is what we're doing here.

Running the AutoMapper-code

Then we run the code using AutoMapper and the mapping of types needing dependency-injection of constructor-parameters:

public static void Run(IContainer container)
{
    var mapper = container.Resolve<IMapper>();

    var source = new EntityType(123, "Test-text", "Test-description");

    var dto = mapper.Map<DtoType>(source);

    dto.RunComponents();
}

The code dto.RunComponents() is the one that executes the injected implementations of the interfaces IPreMapperComponent and IPostMapperComponent used in the type DtoType.

You can find all the code-files involved in this example in this Gist.

This can all also be done through the recommended approach of Modules in Autofac and Profiles in AutoMapper.