Mastering DI Lifetimes

Dependency Injection is the backbone of modern .NET. But if you don't understand Lifetimes, you will create subtle memory leaks and thread-safety bugs that are almost impossible to find.

1. The Three Lifetimes

• Transient: A new instance every time it's requested. Good for lightweight, stateless services.
• Scoped: One instance per HTTP request. Mandatory for Entity Framework DbContexts.
• Singleton: One instance for the life of the app. Good for caches and configs.

2. Captive Dependencies

This is a senior-level mistake. A Captive Dependency occurs when a 'Longer-lived' service holds onto a 'Shorter-lived' service. Example: A **Singleton** service takes a **Scoped** service in its constructor. The Scoped service is now 'Captured' and will live forever. If that Scoped service is a DbContext, you will eventually have a database connection leak that crashes production.

4. Interview Mastery

Q: "How do you solve a Captive Dependency if you truly need a Scoped service in a Singleton?"

Architect Answer: "You use the **IServiceScopeFactory**. Instead of injecting the Scoped service directly, you inject the factory. Inside the Singleton's method, you call `factory.CreateScope()`. You can then resolve the Scoped service from that temporary scope using a `using` block. This ensures the Scoped service is correctly disposed of as soon as your method is finished, while the Singleton continues to live on."