injection-on-demand.md

Injection on demand

This example creates dependencies on demand using a factory delegate. The service (GameLevel) needs multiple instances of IEnemy, so it receives a Func<IEnemy> that can create new instances when needed. This approach is useful when instances are created lazily or repeatedly during business execution.

using Shouldly;
using Pure.DI;
using System.Collections.Generic;

DI.Setup(nameof(Composition))
    .Bind().To<Enemy>()
    .Bind().To<GameLevel>()

    // Composition root
    .Root<IGameLevel>("GameLevel");

var composition = new Composition();
var gameLevel = composition.GameLevel;

// Verifies that two distinct enemies have been spawned
gameLevel.Enemies.Count.ShouldBe(2);

// Represents a game entity that acts as an enemy
interface IEnemy;

class Enemy : IEnemy;

// Represents a game level that manages entities
interface IGameLevel
{
    IReadOnlyList<IEnemy> Enemies { get; }
}

class GameLevel(Func<IEnemy> enemySpawner) : IGameLevel
{
    // The factory spawns a fresh enemy instance on each call.
    public IReadOnlyList<IEnemy> Enemies { get; } =
    [
        enemySpawner(),
        enemySpawner()
    ];
}

Running this code sample locally

• Make sure you have the .NET SDK 10.0 or later installed

dotnet --list-sdk

• Create a net10.0 (or later) console application

dotnet new console -n Sample

• Add references to the NuGet packages
  • Pure.DI
  • Shouldly

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

Key elements:

• Enemy is bound to the IEnemy interface, and GameLevel is bound to IGameLevel.
• The GameLevel constructor accepts Func<IEnemy>, enabling deferred creation of entities.
• The GameLevel calls the factory twice, resulting in two distinct Enemy instances stored in its Enemies collection.

This approach lets factories control lifetime and instantiation timing. Pure.DI resolves a new IEnemy each time the factory is invoked. Limitations: factory delegate calls can create many objects, so lifetime choices still matter for performance and state. Common pitfalls:

• Assuming Func<T> always returns new instances regardless of configured lifetime.
• Hiding expensive work behind repeated on-demand calls. See also: Injections on demand with arguments, Func.

The following partial class will be generated:

partial class Composition
{
  public IGameLevel GameLevel
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      Func<IEnemy> perBlockFunc296 = new Func<IEnemy>(
      [MethodImpl(MethodImplOptions.AggressiveInlining)]
      () =>
      {
        return new Enemy();
      });
      return new GameLevel(perBlockFunc296);
    }
  }
}

Class diagram:

---
 config:
  maxTextSize: 2147483647
  maxEdges: 2147483647
  class:
   hideEmptyMembersBox: true
---
classDiagram
	Enemy --|> IEnemy
	GameLevel --|> IGameLevel
	Composition ..> GameLevel : IGameLevel GameLevel
	GameLevel o-- "PerBlock" FuncᐸIEnemyᐳ : FuncᐸIEnemyᐳ
	FuncᐸIEnemyᐳ *--  Enemy : IEnemy
	namespace Pure.DI.UsageTests.Basics.InjectionOnDemandScenario {
		class Composition {
		<<partial>>
		+IGameLevel GameLevel
		}
		class Enemy {
				<<class>>
			+Enemy()
		}
		class GameLevel {
				<<class>>
			+GameLevel(FuncᐸIEnemyᐳ enemySpawner)
		}
		class IEnemy {
			<<interface>>
		}
		class IGameLevel {
			<<interface>>
		}
	}
	namespace System {
		class FuncᐸIEnemyᐳ {
				<<delegate>>
		}
	}

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