Dependency injection is one way of implementing inversion of control (IoC), which is a design principle that states that an abstraction layer should be used to handle application dependencies. In other words, instead of class A referencing class B, class A should reference an abstraction (i.e., an interface) that class B implements. So, instead of class A depending on class B, class B will depend on an interface that class A controls. This is the inversion of control.
The use of interfaces allows developers to easily swap in different implementations of those interfaces. Thus inversion of control and dependency injection enable you to build applications that are loosely coupled, modular, testable, and maintainable. Note there are many ways to implement dependency injection in ASP.NET Core. While alternative IoC containers such as StructureMap and Ninject provide more advanced features, ASP.NET Core’s built-in DI container is fast and easy to use.
Constructor injection vs. the IServiceProvider interface
Both constructor injection and the IServiceProvider interface are techniques for resolving dependencies. However, they differ significantly in implications and use cases. With constructor injection, the dependencies of a class are passed through the class constructor at compile time. With the IServiceProvider interface, the dependencies are retrieved at run time. Thus, IServiceProvider provides an option when the dependencies are unknown during compilation or may change after compilation.
