Whew! You made it through a lot of content. Let’s review:

• Dependencies in our code can cause problems, like:
  • The codebase is harder to maintain
  • We can’t test classes individually
  • The system isn’t flexible enough to allow us to swap the dependency for something similar
• The Dependency Inversion Principle (DIP) attempts to resolve theses dependency problems by the use of abstraction:
  • “High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g. interfaces).”
  • “Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.”
• Inversion of Control (IoC) is a programming principle that attempts to resolve the dependency problems by giving control to the framework, rather than the dependent class:
  • “Methods defined by the user should be called from within the framework itself, rather than from the user’s application code”
• To apply these principles in our code, we use interfaces and dependency injection
• There are many ways to inject but we used constructor injection
• To do all of this in ASP.NET, we can use the provided IoC container
• We register services with the container by adding “AddX” methods to Startup.ConfigureServices()
• We can register built-in services, like database context, with built-in methods, like:

  public void ConfigureServices(IServiceCollection services)
  {
  services.AddRazorPages();
  services.AddDbContext<SomeContext>();
  }

• We can access some default-registered services, like ILogger<T>, without registering additional services
• We can register custom services, with “AddX” methods, like

  public void ConfigureServices(IServiceProvider services)
  {
  services.AddScoped<IAccount, CustomerAccount>();
  services.AddTransient<IResponder, Responder>();
  services.AddSingleton<ILogger, Logger>();
  }

Sometimes programming principles can feel hard to grasp without actual examples. We provided some in this lesson, but you can find more in these sources:

• “Dependency injection in ASP.NET Core”, Microsoft
• “Understanding Dependency Injection in .NET Core”, Auth0