All 16 Dependency Injection questions
More in Dependency Injection
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? 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 dependency injection, and how does ASP.NET Core's built-in DI container support it?

Beginner

Answer

Dependency injection is a design pattern where a class receives its dependencies (other objects it needs to function) from an external source rather than creating them itself, promoting loose coupling and testability. ASP.NET Core includes a built-in DI container (IServiceProvider) where you register service implementations against interfaces at startup, and the framework automatically resolves and injects the correct implementation wherever a constructor requests that interface.
public interface IEmailSender { Task SendAsync(string to, string body); }
public class SmtpEmailSender : IEmailSender { ... }

builder.Services.AddScoped<IEmailSender, SmtpEmailSender>();

public class OrderService {
    private readonly IEmailSender _emailSender;
    public OrderService(IEmailSender emailSender) => _emailSender = emailSender;  // auto-injected
}
Real-world example A checkout service depends on IEmailSender rather than a concrete SmtpEmailSender, letting unit tests inject a fake implementation and letting the real application swap SMTP providers without changing OrderService's code at all.

Common follow-ups: What are the three built-in service lifetimes and how do they differ?;How does DI relate to the broader Inversion of Control principle?

ASP.NET Core Middleware & Request Pipeline;Testing in .NET (xUnit Integration & Unit Testing)
All 16 Dependency Injection questions