field-injection.md

Field injection

To use dependency injection for a field, make sure the field is writable and simply add the Ordinal attribute to that field, specifying an ordinal that will be used to determine the injection order:

interface IDependency;

class Dependency : IDependency;

interface IService
{
    IDependency? Dependency { get; }
}

class Service : IService
{
    // The Ordinal attribute specifies to perform an injection,
    // the integer value in the argument specifies
    // the ordinal of injection
    [Ordinal(0)] internal IDependency? DependencyVal;

    public IDependency? Dependency => DependencyVal;
}

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

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

var composition = new Composition();
var service = composition.MyService;
service.Dependency.ShouldBeOfType<Dependency>();

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 MyService
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      Service transientService0 = new Service();
      transientService0.DependencyVal = new Dependency();
      return transientService0;
    }
  }
}

Class diagram:

classDiagram
	class Composition {
		<<partial>>
		+IService MyService
	}
	Dependency --|> IDependency
	class Dependency {
		+Dependency()
	}
	Service --|> IService
	class Service {
		+Service()
		~IDependency DependencyVal
	}
	class IDependency {
		<<interface>>
	}
	class IService {
		<<interface>>
	}
	Composition ..> Service : IService MyService
	Service *--  Dependency : IDependency

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