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What is dependency injection, and how does ASP.NET Core's built-in DI container support it? What does GetService<T>() return compared to GetRequiredService<T>() when a service isn't registered? How do you register multiple implementations of the same interface, and how do you resolve all of them together? How does the DI container handle constructor injection when a class has multiple constructors? What is the difference between constructor injection and service location (using IServiceProvider directly to resolve dependencies)? How do you register a service using a factory delegate instead of a simple type mapping, when construction requires custom logic? How do third-party DI containers like Autofac integrate with ASP.NET Core's built-in DI abstractions? How does ASP.NET Core's DI container support injecting a logger via ILogger<T>, and how is the generic type parameter used? What is dependency injection, and why does it make a codebase easier to test and maintain? What is the 'captive dependency' problem, and how does the DI container help catch it?
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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 lifetime, shared across all requests and threads. Scoped creates one instance per client request (or per explicitly created scope), shared within that single request but distinct across different requests. Transient creates a brand-new instance every single time the service is requested, even multiple times within the same request.
builder.Services.AddSingleton<ICacheService, MemoryCacheService>();  // one instance, app lifetime
builder.Services.AddScoped<AppDbContext>();                          // one instance per request
builder.Services.AddTransient<IEmailFormatter, EmailFormatter>();    // new instance every injection
Real-world example A DbContext is registered as Scoped so all repositories within a single HTTP request share the same context (and its change tracking), while a lightweight, stateless formatter is registered as Transient since creating a new instance each time is cheap and avoids any shared-state concerns.

Common follow-ups: What happens if a Singleton service depends on a Scoped service (captive dependency problem)?;How do you choose the right lifetime for a new service?

Configuration & Options;Entity Framework Core & Data Access
All 16 Dependency Injection questions