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What is dependency injection, and what problem does it solve compared to a class creating its own dependencies directly? What is the difference between Singleton, Scoped, and Transient service lifetimes in ASP.NET Core's DI container? What is the Inversion of Control (IoC) principle, and how does dependency injection relate to it as one specific technique? How does constructor injection differ from property (setter) injection, and why is constructor injection generally preferred? What causes a 'captive dependency' bug, and how does ASP.NET Core's DI container detect and prevent it? How would you implement the Factory pattern using dependency injection to resolve a specific implementation at runtime based on a parameter, when the standard container can't decide statically? How do you correctly and safely resolve a Scoped service from within a Singleton service using IServiceScopeFactory, avoiding the captive dependency problem? How would you write a custom IServiceProviderFactory or use a third-party container (like Autofac) to enable advanced DI features not supported by the built-in ASP.NET Core container, like property injection or convention-based registration? How does the DI container resolve a class with MULTIPLE constructors, and how do you influence which one gets chosen?
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How do you register and resolve a service using ASP.NET Core's built-in dependency injection container?

Beginner

Answer

Register a service's interface-to-implementation mapping in Program.cs using builder.Services.AddXxx(), then ASP.NET Core automatically INJECTS it into any controller or class constructor that declares a dependency on that interface — you never call 'new' or manually resolve it yourself.
// Program.cs
builder.Services.AddScoped<IOrderService, OrderService>();

// Automatically injected into a controller's constructor
public class OrdersController : ControllerBase {
  private readonly IOrderService _orderService;
  public OrdersController(IOrderService orderService) { _orderService = orderService; }
}
Real-world example Registering a repository or service interface in an ASP.NET Core web API's startup configuration.

Common follow-ups: What happens if you inject a service into a constructor but forget to register it in the DI container?

Interfaces & Abstract Classes
All 10 Dependency Injection & IoC Principles questions