span-and-readonlyspan.md

Span and ReadOnlySpan

Specifying Span<T> and ReadOnlySpan<T> work the same as with the array T[].

struct Dependency;

interface IService
{
    int Count { get; }
}

class Service(ReadOnlySpan<Dependency> dependencies) : IService
{
    public int Count { get; } = dependencies.Length;
}

DI.Setup(nameof(Composition))
    .Bind<Dependency>('a').To<Dependency>()
    .Bind<Dependency>('b').To<Dependency>()
    .Bind<Dependency>('c').To<Dependency>()
    .Bind<IService>().To<Service>()

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

var composition = new Composition();
var service = composition.Root;
service.Count.ShouldBe(3);

This scenario is even more efficient in the case of Span<T> or ReadOnlySpan<T> when T is a value type. In this case, there is no heap allocation, and the composition root IService looks like this:

public IService Root
{
  get
  {
    ReadOnlySpan<Dependency> dependencies = stackalloc Dependency[3] { new Dependency(), new Dependency(), new Dependency() };
    return new Service(dependencies);
  }
}

The following partial class will be generated:

partial class Composition
{
  private readonly Composition _root;

  public Composition()
  {
    _root = this;
  }

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

  public IService Root
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      ReadOnlySpan<Dependency> transientReadOnlySpan1 = stackalloc Dependency[3]
      {
          new Dependency(),
          new Dependency(),
          new Dependency()
      };
      return new Service(transientReadOnlySpan1);
    }
  }

  [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.Root;
    }

    public override IService ResolveByTag(Composition composition, object tag)
    {
      switch (tag)
      {
        case null:
          return composition.Root;

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

Class diagram:

classDiagram
	class Composition {
		<<partial>>
		+IService Root
		+ T ResolveᐸTᐳ()
		+ T ResolveᐸTᐳ(object? tag)
		+ object Resolve(Type type)
		+ object Resolve(Type type, object? tag)
	}
	class Dependency {
		+Dependency()
	}
	Service --|> IService
	class Service {
		+Service(ReadOnlySpanᐸDependencyᐳ dependencies)
	}
	class ReadOnlySpanᐸDependencyᐳ
	class IService {
		<<interface>>
	}
	Service *--  ReadOnlySpanᐸDependencyᐳ : ReadOnlySpanᐸDependencyᐳ
	Composition ..> Service : IService Root
	ReadOnlySpanᐸDependencyᐳ *--  Dependency : 'a'  Dependency
	ReadOnlySpanᐸDependencyᐳ *--  Dependency : 'b'  Dependency
	ReadOnlySpanᐸDependencyᐳ *--  Dependency : 'c'  Dependency