Service Lifetimes in .NET Core (Transient vs Scoped vs Singleton)
💡 Concept Name
Service Lifetimes in Dependency Injection
📘 Quick Intro
.NET Core offers three types of service lifetimes for Dependency Injection: Transient, Scoped, and Singleton. Each controls how long an instance of a service lives.
🧠 Analogy / Short Story
Imagine a water bottle:
- Transient: A new bottle every time you ask.
- Scoped: One bottle per trip to the water cooler.
- Singleton: One bottle shared by everyone forever.
🔧 Technical Explanation
- Transient: A new instance every time it's requested.
- Scoped: A new instance per HTTP request (per scope).
- Singleton: A single shared instance for the entire application lifetime.
🎯 Purpose & Use Case
- ✅ Transient: Use when the service has no state or is cheap to create.
- ✅ Scoped: Use for services that need to track request-specific data.
- ✅ Singleton: Use for shared services or global state like cache, config providers.
💻 Real Code Example
// Registering services
builder.Services.AddTransient<IMyService, MyService>();
builder.Services.AddScoped<IMyService, MyService>();
builder.Services.AddSingleton<IMyService, MyService>();
// Injected into controllers
public class HomeController : Controller
{
private readonly IMyService _service;
public HomeController(IMyService service)
{
_service = service;
}
}
❓ Interview Q&A
Q1: What are the three service lifetimes?
A: Transient, Scoped, Singleton.
Q2: What does Transient mean?
A: A new instance is created every time.
Q3: When is Scoped instance created?
A: Once per HTTP request or scope.
Q4: How long does a Singleton last?
A: For the entire lifetime of the application.
Q5: Which lifetime is ideal for shared cache?
A: Singleton.
Q6: Can Singleton services have constructor dependencies?
A: Yes, but those must also be Singleton or safe to share.
Q7: What happens if Scoped is injected into Singleton?
A: It can cause runtime errors or memory leaks.
Q8: Which method registers a Transient service?
A: AddTransient.
Q9: Which method registers a Singleton service?
A: AddSingleton.
Q10: Which service lifetime is recommended for DB context?
A: Scoped.
📝 MCQs
Q1. Which lifetime creates a new instance each time it's requested?
- Scoped
- Singleton
- Transient
- Static
Q2. What lifetime provides the same instance within a request?
- Transient
- Singleton
- Scoped
- Global
Q3. Which lifetime lives for the whole app runtime?
- Transient
- Scoped
- Singleton
- Thread
Q4. Which method registers a Singleton service?
- AddScoped
- AddTransient
- AddSingleton
- UseSingleton
Q5. When should you use Scoped services?
- Always
- When performance needed
- When per-request instance is needed
- To reduce memory
Q6. What happens when a Scoped service is injected into Singleton?
- Nothing
- Scoped becomes Singleton
- It can cause runtime issues
- It logs a warning only
Q7. Which DI lifetime is least memory efficient if used incorrectly?
- Scoped
- Transient
- Singleton
- None of these
Q8. Which of these is ideal for stateless lightweight services?
- Singleton
- Transient
- Scoped
- Static
Q9. Where do you register service lifetimes?
- Startup.cs Main method
- Web.config
- Controller constructor
- builder.Services in Program.cs
Q10. What’s the correct syntax for Scoped service registration?
- AddService()
- AddScoped()
- RegisterScoped()
- InjectScoped()
💡 Bonus Insight
Never inject a Scoped service into a Singleton service. Use interfaces to reduce tight coupling and enable easy lifetime switching.
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