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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? What is the difference between Singleton, Scoped, and Transient service lifetimes in ASP.NET Core's DI container? How do you register multiple implementations of the same interface, and how do you resolve all of them together? 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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How does the DI container handle constructor injection when a class has multiple constructors?

Intermediate

Answer

The default ASP.NET Core DI container requires exactly one constructor be resolvable (all its parameter types must be registered services) -- if multiple constructors could theoretically be satisfied, or if the class has multiple public constructors at all in some scenarios, the container throws an exception rather than guessing which one to use, since implicit 'pick the best constructor' behavior (common in other DI frameworks) is intentionally not supported by the built-in container to avoid ambiguity.
public class OrderService {
    // Having two constructors like this causes issues with the built-in container:
    public OrderService(IEmailSender emailSender) { ... }
    public OrderService(IEmailSender emailSender, ILogger<OrderService> logger) { ... }
    // Built-in container: ambiguous, throws at resolution time
}
// Fix: use exactly one constructor, or use a third-party container supporting this
Real-world example A team migrating from a third-party DI container (which supported multiple constructor overloads with fallback resolution) to the built-in ASP.NET Core container hits runtime errors until they consolidate each service down to a single constructor.

Common follow-ups: Which third-party containers (Autofac, etc.) do support multiple constructor resolution?;What's the recommended pattern instead of multiple constructors?

Dependency Injection;Diagnostics & Performance
All 16 Dependency Injection questions