lazy.md

July 3, 2026 · View on GitHub

Lazy

Injecting Lazy<T> defers creation of a dependency until its Value property is first accessed, after which the same instance is returned every time. No extra setup is needed: bind the underlying type as usual and request Lazy<T> in the constructor.

using Shouldly;
using Pure.DI;

DI.Setup(nameof(Composition))
    .Bind<IGraphicsEngine>().To<GraphicsEngine>()
    .Bind<IWindow>().To<Window>()

    // Composition root
    .Root<IWindow>("Window");

var composition = new Composition();
var window = composition.Window;

// The graphics engine is created only when it is first accessed
window.Engine.ShouldBe(window.Engine);

interface IGraphicsEngine;

class GraphicsEngine : IGraphicsEngine;

interface IWindow
{
    IGraphicsEngine Engine { get; }
}

class Window(Lazy<IGraphicsEngine> engine) : IWindow
{
    public IGraphicsEngine Engine => engine.Value;
}
Running this code sample locally 
dotnet --list-sdk
  • Create a net10.0 (or later) console application
dotnet new console -n Sample

dotnet add package Pure.DI
dotnet add package Shouldly
  • Copy the example code into the Program.cs file

You are ready to run the example 🚀

dotnet run

Note

Lazy is useful for expensive-to-create objects or when the instance may never be needed, improving application startup performance.

The following partial class will be generated 
partial class Composition
{
  public IWindow Window
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      Lazy<IGraphicsEngine> transientLazyIGraphicsEngine;
      // Creates a lazy value factory
      Func<IGraphicsEngine> perBlockFuncIGraphicsEngine = new Func<IGraphicsEngine>(
      [MethodImpl(MethodImplOptions.AggressiveInlining)]
      () =>
      {
        // Creates a deferred value
        return new GraphicsEngine();
      });
      Func<IGraphicsEngine> localFactory = perBlockFuncIGraphicsEngine;
      // Wraps it in Lazy<T>
      transientLazyIGraphicsEngine = new Lazy<IGraphicsEngine>(localFactory, true);
      return new Window(transientLazyIGraphicsEngine);
    }
  }
}

Class diagram:

---
 config:
  class:
   hideEmptyMembersBox: true
---
classDiagram
	GraphicsEngine --|> IGraphicsEngine
	Window --|> IWindow
	Composition ..> Window : IWindow Window
	Window *-- LazyᐸIGraphicsEngineᐳ : LazyᐸIGraphicsEngineᐳ
	LazyᐸIGraphicsEngineᐳ o-- "PerBlock" FuncᐸIGraphicsEngineᐳ : FuncᐸIGraphicsEngineᐳ
	FuncᐸIGraphicsEngineᐳ *-- GraphicsEngine : IGraphicsEngine
	namespace Pure.DI.UsageTests.BCL.LazyScenario {
		class Composition {
		<<partial>>
		+IWindow Window
		}
		class GraphicsEngine {
				<<class>>
			+GraphicsEngine()
		}
		class IGraphicsEngine {
			<<interface>>
		}
		class IWindow {
			<<interface>>
		}
		class Window {
				<<class>>
			+Window(LazyᐸIGraphicsEngineᐳ engine)
		}
	}
	namespace System {
		class FuncᐸIGraphicsEngineᐳ {
				<<delegate>>
		}
		class LazyᐸIGraphicsEngineᐳ {
				<<class>>
		}
	}

See also: