IServiceCollection

IServiceCollection is a fundamental interface in the .NET dependency injection (DI) system. It represents a collection of service descriptors, which define how services should be instantiated and managed. Essentially, it acts as a registry where you register different types of services and specify their lifetimes.

When you configure dependency injection in a .NET application, you typically start with an instance of IServiceCollection. This collection is then passed to various extension methods that add pre-defined services or allow you to register your own custom services. For example, you might register a class with a transient, scoped, or singleton lifetime.

Transient services are created every time they are requested. Scoped services are created once per client request. Singleton services are created once and shared across the entire application. The IServiceCollection stores these registration details, enabling the DI container to resolve dependencies correctly when they are needed.

The `IServiceCollection` itself does not perform the service resolution. Instead, it is used to build an `IServiceProvider`. The `IServiceProvider` is the object that actually fulfills dependency requests by creating and providing instances of the registered services. Therefore, `IServiceCollection` is primarily used during the application's startup or configuration phase to define the DI graph.