Choosing the right dependency injection (DI) lifetime is essential for the performance, stability, and maintainability of .NET applications. This guide breaks down the three main DI lifetimes—Transient, Scoped, and Singleton—to help you make informed decisions.
Transient: New Instance Every Time
Use Case: Lightweight, stateless services.
Behavior: A new instance is created every time it is requested.
Examples:
- Repositories
- Email senders
- Logging services
Pros:
- Ensures the service is always fresh and not affected by previous states.
- Avoids unintended shared states between requests.
Cons:
- Excessive use may lead to increased memory usage and unnecessary object creation.
Scoped: One Instance Per Request
Use Case: Services tied to a request lifecycle.
Behavior: A single instance is created per HTTP request and shared within that request.
Examples:
- Entity Framework (EF) Core DbContext
- Business logic services that rely on request-specific data
Pros:
- Ensures consistency within a single request while preventing memory bloat.
- Ideal for database contexts and business logic that should not be shared across requests.
Cons:
- Injecting scoped services into singleton services can cause unexpected behaviors.
- Not suitable for cross-request dependencies.
Best Practice: Never inject scoped services into singleton services as it can lead to memory leaks and unintended data persistence.
Singleton: One Instance for the Application Lifetime
Use Case: Heavy, shared resources that should not be frequently recreated.
Behavior: A single instance is created when the application starts and remains for its entire lifetime.
Examples:
- Caching services
- Configuration management
- Logging
- Static data
Pros:
- Reduces object creation overhead and improves performance.
- Ensures a consistent global state.
Cons:
- Risk of memory leaks if objects hold unnecessary state.
- Must be thread-safe to avoid concurrency issues.
- Avoid injecting transient or scoped services directly into singletons.
Avoid: Singleton services depending on scoped/transient services, as they may reference disposed objects or cause unintended side effects.
Avoiding Service Lifetime Mismatches
✅ Good Practice:
- Singleton -> Uses other singletons.
- Scoped -> Uses scoped or transient services.
- Transient -> Uses other transient services.
❌ Bad Practice:
- Singleton -> Uses Scoped/Transient services (may lead to lifecycle mismatches and disposed dependencies).
Conclusion
Understanding the correct DI lifetime for your services helps optimize performance and ensures stability in .NET applications. Use Transient for lightweight stateless services, Scoped for per-request dependencies, and Singleton for long-lived shared resources. Always be mindful of service lifetime mismatches to prevent issues like memory leaks and unintended behaviors.
References
- Microsoft Documentation – Dependency injection in .NET
Official documentation explaining the DI container, lifetimes, and best practices. - Microsoft Docs – Service lifetimes in Dependency Injection
Detailed breakdown of Singleton, Scoped, and Transient lifetimes.