onnewinstance-hint.md

OnNewInstance hint

Hints are used to fine-tune code generation. The OnNewInstance hint determines whether to generate partial OnNewInstance method. In addition, setup hints can be comments before the Setup method in the form hint = value, for example: // OnNewInstance = On.

using static Hint;

interface IDependency;

class Dependency : IDependency
{
    public override string ToString() => nameof(Dependency);
}

interface IService
{
    IDependency Dependency { get; }
}

class Service(IDependency dependency) : IService
{
    public IDependency Dependency { get; } = dependency;

    public override string ToString() => nameof(Service);
}

internal partial class Composition
{
    private readonly List<string> _log = [];

    public Composition(List<string> log) : this() =>
        _log = log;

    partial void OnNewInstance<T>(
        ref T value,
        object? tag,
        Lifetime lifetime) =>
        _log.Add(typeof(T).Name);
}

DI.Setup(nameof(Composition))
    .Hint(OnNewInstance, "On")
    .Bind().As(Lifetime.Singleton).To<Dependency>()
    .Bind().To<Service>()
    .Root<IService>("Root");

var log = new List<string>();
var composition = new Composition(log);
var service1 = composition.Root;
var service2 = composition.Root;

log.ShouldBe([nameof(Dependency), nameof(Service), nameof(Service)]);

The OnNewInstanceLifetimeRegularExpression hint helps you define a set of lifetimes that require instance creation control. You can use it to specify a regular expression to filter bindings by lifetime name. For more hints, see this page.

The following partial class will be generated:

partial class Composition
{
  private readonly Composition _root;
  private readonly Lock _lock;

  private Dependency? _singletonDependency43;

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

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

  public IService Root
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      if (_root._singletonDependency43 is null)
      {
        using (_lock.EnterScope())
        {
          if (_root._singletonDependency43 is null)
          {
            Dependency _singletonDependency43Temp;
            _singletonDependency43Temp = new Dependency();
            OnNewInstance<Dependency>(ref _singletonDependency43Temp, null, Lifetime.Singleton);
            Thread.MemoryBarrier();
            _root._singletonDependency43 = _singletonDependency43Temp;
          }
        }
      }

      Service transientService0 = new Service(_root._singletonDependency43!);
      OnNewInstance<Service>(ref transientService0, null, Lifetime.Transient);
      return transientService0;
    }
  }


  partial void OnNewInstance<T>(ref T value, object? tag, Lifetime lifetime);
}

Class diagram:

classDiagram
	class Composition {
		<<partial>>
		+IService Root
	}
	Service --|> IService
	class Service {
		+Service(IDependency dependency)
	}
	Dependency --|> IDependency
	class Dependency {
		+Dependency()
	}
	class IService {
		<<interface>>
	}
	class IDependency {
		<<interface>>
	}
	Composition ..> Service : IService Root
	Service o-- "Singleton" Dependency : IDependency

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