.NET Dependency Injection

Dependency Injection (DI) is a first-class citizen in the .NET ecosystem, particularly since the introduction of the Generic Host. It is a design pattern used to achieve Inversion of Control (IoC) between classes and their dependencies.

Core Concepts

1. Service Collection

The container where services are registered during application startup.

var builder = Host.CreateApplicationBuilder(args);
builder.Services.AddSingleton<IMyService, MyService>();

2. Service Provider

The object that resolves and provides instances of registered services.

3. Service Lifetimes

Transient: A new instance is created every time it is requested.
Scoped: A new instance is created once per client request (within a scope).
Singleton: A single instance is created the first time it is requested and used by all subsequent requests.

The Generic Host

Modern .NET applications (Console, Web, Worker) use the Generic Host to manage application lifetime, configuration, logging, and DI.

Background Services: Classes that inherit from BackgroundService can be registered to run long-running tasks.

Why it Matters for Agents

DI is essential for building modular agentic frameworks:

Mocking: Easily swap a live ILLMService with a mock during testing.
Configuration: Inject agent settings (API keys, model names) into the classes that need them.
Extensibility: Allow users to register custom "Skills" or "Tools" that the agent can then resolve via DI.