singleton.md

Singleton

The Singleton lifetime ensures that there will be a single instance of the dependency for each composition.

interface IDependency;

class Dependency : IDependency;

interface IService
{
    public IDependency Dependency1 { get; }

    public IDependency Dependency2 { get; }
}

class Service(
    IDependency dependency1,
    IDependency dependency2)
    : IService
{
    public IDependency Dependency1 { get; } = dependency1;

    public IDependency Dependency2 { get; } = dependency2;
}

DI.Setup(nameof(Composition))
    // This hint indicates to not generate methods such as Resolve
    .Hint(Hint.Resolve, "Off")
    .Bind().As(Lifetime.Singleton).To<Dependency>()
    .Bind().To<Service>()
    .Root<IService>("Root");

var composition = new Composition();
var service1 = composition.Root;
var service2 = composition.Root;
service1.Dependency1.ShouldBe(service1.Dependency2);
service2.Dependency1.ShouldBe(service1.Dependency1);

Some articles advise using objects with a Singleton lifetime as often as possible, but the following details must be considered:

• For .NET the default behavior is to create a new instance of the type each time it is needed, other behavior requires, additional logic that is not free and requires additional resources.

• The use of Singleton, adds a requirement for thread-safety controls on their use, since singletons are more likely to share their state between different threads without even realizing it.

• The thread-safety control should be automatically extended to all dependencies that Singleton uses, since their state is also now shared.

• Logic for thread-safety control can be resource-costly, error-prone, interlocking, and difficult to test.

• Singleton can retain dependency references longer than their expected lifetime, this is especially significant for objects that hold "non-renewable" resources, such as the operating system Handler.

• Sometimes additional logic is required to dispose of Singleton.

The following partial class will be generated:

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

  private Dependency? _singletonDependency43;

  [OrdinalAttribute(256)]
  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)
          {
            _root._singletonDependency43 = new Dependency();
          }
        }
      }

      return new Service(_root._singletonDependency43!, _root._singletonDependency43!);
    }
  }
}

Class diagram:

---
 config:
  class:
   hideEmptyMembersBox: true
---
classDiagram
	Service --|> IService
	Dependency --|> IDependency
	Composition ..> Service : IService Root
	Service o-- "2 Singleton instances" Dependency : IDependency
	namespace Pure.DI.UsageTests.Lifetimes.SingletonScenario {
		class Composition {
		<<partial>>
		+IService Root
		}
		class Dependency {
			+Dependency()
		}
		class IDependency {
			<<interface>>
		}
		class IService {
			<<interface>>
		}
		class Service {
			+Service(IDependency dependency1, IDependency dependency2)
		}
	}

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