# Service Lifetimes — .NET Core: Build, Test and Ship Web APIs

Source: https://www.geekswithgeeks.com/en/dotnet-core/di-lifetimes

> Choose between transient, scoped and singleton and avoid the captive dependency bug.

## Three lifetimes

**Transient**: a new instance every time. **Scoped**: one instance per HTTP request. **Singleton**: one instance for the whole app. Database contexts are scoped; stateless helpers and caches are often singletons.

## Registering each lifetime

Pick the shortest lifetime that works. A singleton must be thread-safe because many requests share it.

```csharp
builder.Services.AddTransient<IIdGenerator, IdGenerator>();
builder.Services.AddScoped<IOrderService, OrderService>();
builder.Services.AddSingleton<IClock, SystemClock>();
```

## Captive dependency

A singleton must not hold a scoped service such as `DbContext`: it would keep one request's instance alive forever. In development, scope validation catches this at startup.

**Quiz:** How many instances of a scoped service exist during one HTTP request?

- [ ] A new one per use
- [ ] Zero
- [x] One
- [ ] One per thread forever

*Answer:* One. Scoped services are created once per request scope and shared within it.