Why use dependency injection? Explain constructor injection and the benefits it brings to an ASP.NET Core app.

Dependency injection means a class receives its dependencies from outside instead of creating them. With constructor injection the dependencies are constructor parameters, usually interfaces, for example OrderService(IOrderRepository repo, ILogger<OrderService> log). The container builds the object graph from registrations like builder.Services.AddScoped<IOrderRepository, SqlOrderRepository>().

Benefits: loose coupling (code depends on abstractions), easy unit testing (pass fakes or mocks), central control of lifetimes (transient, scoped, singleton) and disposal, swapping implementations through configuration, and explicit dependencies that are visible in the constructor.

Key points:
- Dependencies are provided from outside rather than created with new
- Constructor injection: dependencies are constructor parameters, typically interfaces
- Services are registered in the container with a lifetime
- Improves testability: you can inject fakes or mocks
- Loose coupling: implementations can be swapped