type-attribute.md

Type attribute

The injection type can be defined manually using the Type attribute. This attribute explicitly overrides an injected type, otherwise it would be determined automatically based on the type of the constructor/method, property, or field parameter.

interface IDependency;

class AbcDependency : IDependency;

class XyzDependency : IDependency;

interface IService
{
    IDependency Dependency1 { get; }

    IDependency Dependency2 { get; }
}

class Service(
    [Type(typeof(AbcDependency))] IDependency dependency1,
    [Type(typeof(XyzDependency))] IDependency dependency2)
    : IService
{
    public IDependency Dependency1 { get; } = dependency1;

    public IDependency Dependency2 { get; } = dependency2;
}

DI.Setup(nameof(Composition))
    .Bind().To<Service>()

    // Composition root
    .Root<IService>("Root");

var composition = new Composition();
var service = composition.Root;
service.Dependency1.ShouldBeOfType<AbcDependency>();
service.Dependency2.ShouldBeOfType<XyzDependency>();

This attribute is part of the API, but you can use your own attribute at any time, and this allows you to define them in the assembly and namespace you want.

The following partial class will be generated:

partial class Composition
{
  private readonly Composition _root;

  [OrdinalAttribute(20)]
  public Composition()
  {
    _root = this;
  }

  internal Composition(Composition parentScope)
  {
    _root = (parentScope ?? throw new ArgumentNullException(nameof(parentScope)))._root;
  }

  public IService Root
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new Service(new AbcDependency(), new XyzDependency());
    }
  }
}

Class diagram:

classDiagram
	class Composition {
		<<partial>>
		+IService Root
	}
	class XyzDependency {
		+XyzDependency()
	}
	class AbcDependency {
		+AbcDependency()
	}
	Service --|> IService
	class Service {
		+Service(IDependency dependency1, IDependency dependency2)
	}
	class IService {
		<<interface>>
	}
	Composition ..> Service : IService Root
	Service *--  AbcDependency : AbcDependency
	Service *--  XyzDependency : XyzDependency

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