Dev Blog

Summary ​

Conclusions ​

Dependency injection makes classes independent of concrete implementations: a class declares the interfaces it needs in its constructor, and the objects are wired together in the composition root. The Microsoft.Extensions.DependencyInjection container creates services from registrations, supports multiple implementations, factories, and keyed services, and manages the Singleton, Scoped, and Transient lifetimes; the ValidateScopes and ValidateOnBuild checks detect captive and missing dependencies. The Generic Host combines DI, configuration, logging, and background services and shuts down gracefully on Ctrl+C. Configuration is assembled from JSON files, user secrets, environment variables, and the command line, with later sources overriding earlier ones. The options pattern provides typed access to settings that is validated at startup. The ILogger<T> logger with levels, categories, and message templates writes structured data to several providers, and the [LoggerMessage] generator makes it fast. The same techniques work in Windows Forms and simplify unit testing.

Self-check questions ​

  1. What are a dependency and tight coupling? What problems does new of a concrete class inside a service create?
  2. State the dependency inversion principle. What is constructor injection?
  3. What is a composition root? Why is the service locator considered an antipattern?
  4. What are IServiceCollection and IServiceProvider for? How does GetService differ from GetRequiredService?
  5. What does the container return if one interface is registered twice? How do you get all implementations?
  6. What are keyed services and the [FromKeyedServices] attribute for?
  7. How do the Singleton, Scoped, and Transient lifetimes differ? When does the container release objects?
  8. What is a captive dependency? How do you detect it?
  9. What does Host.CreateApplicationBuilder configure? How does BackgroundService work?
  10. Which configuration providers does the host add, and in what order of priority?
  11. How do you set the runtime environment? What is the launchSettings.json file for?
  12. How do you write a hierarchical key in an environment variable and in a command-line argument?
  13. Why are user secrets needed? Where are they stored?
  14. How do IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T> differ? How do you validate options at startup?
  15. What log levels exist? What is a log category?
  16. Why do you pass a message template rather than an interpolated string to the Log… methods?
  17. Which built-in logging providers does .NET have? How do you configure filters in appsettings.json?
  18. How do you add the host and dependency injection to a Windows Forms application?