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What is dependency injection, and what problem does it solve compared to a class creating its own dependencies directly? How do you register and resolve a service using ASP.NET Core's built-in dependency injection container? What is the Inversion of Control (IoC) principle, and how does dependency injection relate to it as one specific technique? How does constructor injection differ from property (setter) injection, and why is constructor injection generally preferred? What causes a 'captive dependency' bug, and how does ASP.NET Core's DI container detect and prevent it? How would you implement the Factory pattern using dependency injection to resolve a specific implementation at runtime based on a parameter, when the standard container can't decide statically? How do you correctly and safely resolve a Scoped service from within a Singleton service using IServiceScopeFactory, avoiding the captive dependency problem? How would you write a custom IServiceProviderFactory or use a third-party container (like Autofac) to enable advanced DI features not supported by the built-in ASP.NET Core container, like property injection or convention-based registration? How does the DI container resolve a class with MULTIPLE constructors, and how do you influence which one gets chosen?
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What is the difference between Singleton, Scoped, and Transient service lifetimes in ASP.NET Core's DI container?

Intermediate

Answer

Singleton creates ONE instance for the entire application's lifetime, shared across all requests; Scoped creates ONE instance PER REQUEST (or per logical scope), shared within that single request but recreated for the next; Transient creates a BRAND NEW instance every single time the service is requested/injected, even multiple times within the same request.
builder.Services.AddSingleton<ICacheService, CacheService>();   // one instance, app-wide
builder.Services.AddScoped<IOrderService, OrderService>();       // one instance per HTTP request
builder.Services.AddTransient<IEmailBuilder, EmailBuilder>();    // new instance every injection
Real-world example Registering a shared in-memory cache as Singleton, a per-request database context as Scoped, and a lightweight stateless helper as Transient.

Common follow-ups: What bug (a 'captive dependency') occurs if a Singleton service depends on a Scoped service?

Fundamentals
All 10 Dependency Injection & IoC Principles questions