Dependency Injection is one of the core concepts of modern ASP.NET Core applications. It means that a class receives its dependencies from outside instead of creating them internally. This makes the code easier to test, extend and maintain.

Creating services or database contexts directly inside controllers may look simple at first, but it quickly creates tight coupling as the application grows. With Dependency Injection, dependencies are provided through constructors and their creation rules are configured during application startup.

In ASP.NET Core, services are commonly registered with AddScoped, AddTransient or AddSingleton. Scoped services are created once per HTTP request and are commonly used for DbContext. Transient services are created every time they are requested. Singleton services live for the entire application lifetime.

Choosing the correct lifetime is important for performance and data consistency. For example, registering DbContext as Singleton can cause dangerous shared state problems between requests. Service lifetimes should always be selected according to the behavior of the dependency.

Dependency Injection is not just a convenience feature. It is also a key part of clean architecture. Repositories, application services, email senders, file uploaders and slug generators become easier to manage when they are registered and consumed through DI.