resolve-methods.md

Resolve methods

This example shows how to resolve the composition roots using the Resolve methods by Service Locator approach. Resolve methods are generated automatically for each registered root.

interface IDependency;

class Dependency : IDependency;

interface IService;

class Service(IDependency dependency) : IService;

class OtherService : IService;

DI.Setup(nameof(Composition))
    .Bind<IDependency>().To<Dependency>()
    .Bind<IService>().To<Service>()
    .Bind<IService>("My Tag").To<OtherService>()

    // Specifies to create a private root
    // that is only accessible from _Resolve_ methods
    .Root<IService>()

    // Specifies to create a public root named _OtherService_
    // using the "My Tag" tag
    .Root<IService>("OtherService", "My Tag");

var composition = new Composition();

// The next 3 lines of code do the same thing:
var service1 = composition.Resolve<IService>();
var service2 = composition.Resolve(typeof(IService));
var service3 = composition.Resolve(typeof(IService), null);

// Resolve by "My Tag" tag
// The next 3 lines of code do the same thing too:
var otherService1 = composition.Resolve<IService>("My Tag");
var otherService2 = composition.Resolve(typeof(IService), "My Tag");
var otherService3 = composition.OtherService; // Gets the composition through the public root

Resolve methods are similar to calls to the roots of a composition. Composition roots are common properties. Their use is efficient and does not cause exceptions. And that is why it is recommended to use them. In contrast, Resolve methods have a number of disadvantages:

• They provide access to an unlimited set of dependencies.

• Their use can potentially lead to runtime exceptions, for example, when the corresponding root has not been defined.

• Lead to performance degradation because they search for the root of a composition based on its type.

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 OtherService
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new OtherService();
    }
  }

  private IService Root2
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new Service(new Dependency());
    }
  }

  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public T Resolve<T>()
  {
    return Resolver<T>.Value.Resolve(this);
  }

  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public T Resolve<T>(object? tag)
  {
    return Resolver<T>.Value.ResolveByTag(this, tag);
  }

  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public object Resolve(Type type)
  {
    var index = (int)(_bucketSize * ((uint)RuntimeHelpers.GetHashCode(type) % 1));
    ref var pair = ref _buckets[index];
    return pair.Key == type ? pair.Value.Resolve(this) : Resolve(type, index);
  }

  [MethodImpl(MethodImplOptions.NoInlining)]
  private object Resolve(Type type, int index)
  {
    var finish = index + _bucketSize;
    while (++index < finish)
    {
      ref var pair = ref _buckets[index];
      if (pair.Key == type)
      {
        return pair.Value.Resolve(this);
      }
    }

    throw new InvalidOperationException($"{CannotResolveMessage} {OfTypeMessage} {type}.");
  }

  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public object Resolve(Type type, object? tag)
  {
    var index = (int)(_bucketSize * ((uint)RuntimeHelpers.GetHashCode(type) % 1));
    ref var pair = ref _buckets[index];
    return pair.Key == type ? pair.Value.ResolveByTag(this, tag) : Resolve(type, tag, index);
  }

  [MethodImpl(MethodImplOptions.NoInlining)]
  private object Resolve(Type type, object? tag, int index)
  {
    var finish = index + _bucketSize;
    while (++index < finish)
    {
      ref var pair = ref _buckets[index];
      if (pair.Key == type)
      {
        return pair.Value.ResolveByTag(this, tag);
      }
    }

    throw new InvalidOperationException($"{CannotResolveMessage} \"{tag}\" {OfTypeMessage} {type}.");
  }

  private readonly static int _bucketSize;
  private readonly static Pair<Type, IResolver<Composition, object>>[] _buckets;

  static Composition()
  {
    var valResolver_0000 = new Resolver_0000();
    Resolver<IService>.Value = valResolver_0000;
    _buckets = Buckets<Type, IResolver<Composition, object>>.Create(
      1,
      out _bucketSize,
      new Pair<Type, IResolver<Composition, object>>[1]
      {
         new Pair<Type, IResolver<Composition, object>>(typeof(IService), valResolver_0000)
      });
  }

  private const string CannotResolveMessage = "Cannot resolve composition root ";
  private const string OfTypeMessage = "of type ";

  private class Resolver<T>: IResolver<Composition, T>
  {
    public static IResolver<Composition, T> Value = new Resolver<T>();

    public virtual T Resolve(Composition composite)
    {
      throw new InvalidOperationException($"{CannotResolveMessage}{OfTypeMessage}{typeof(T)}.");
    }

    public virtual T ResolveByTag(Composition composite, object tag)
    {
      throw new InvalidOperationException($"{CannotResolveMessage}\"{tag}\" {OfTypeMessage}{typeof(T)}.");
    }
  }

  private sealed class Resolver_0000: Resolver<IService>
  {
    public override IService Resolve(Composition composition)
    {
      return composition.Root2;
    }

    public override IService ResolveByTag(Composition composition, object tag)
    {
      switch (tag)
      {
        case "My Tag":
          return composition.OtherService;

        case null:
          return composition.Root2;

        default:
          return base.ResolveByTag(composition, tag);
      }
    }
  }
}

Class diagram:

classDiagram
	class Composition {
		<<partial>>
		+IService OtherService
		-IService _
		+ T ResolveᐸTᐳ()
		+ T ResolveᐸTᐳ(object? tag)
		+ object Resolve(Type type)
		+ object Resolve(Type type, object? tag)
	}
	Dependency --|> IDependency
	class Dependency {
		+Dependency()
	}
	Service --|> IService
	class Service {
		+Service(IDependency dependency)
	}
	OtherService --|> IService : "My Tag" 
	class OtherService {
		+OtherService()
	}
	class IDependency {
		<<interface>>
	}
	class IService {
		<<interface>>
	}
	Composition ..> OtherService : IService OtherService
	Composition ..> Service : IService _
	Service *--  Dependency : IDependency

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