.NET Core  

Service Lifetimes in ASP.NET Core: Transient vs Scoped vs Singleton

Introduction to Service Lifetimes in .NET

In ASP.NET Core, Dependency Injection (DI) is a powerful feature that helps developers build loosely coupled, testable, and maintainable applications. One key aspect of this system is understanding how services are managed over time, which is where Service Lifetimes come into play.

A Service Lifetime determines how long an instance of a service will live once it's created by the built-in IoC (Inversion of Control) container.

When a service is registered in the Program.cs (or Startup.cs in older versions), we must specify its lifetime. This decision directly affects.

  • When the service instance is created
  • How long will it be reused
  • Whether it's shared across requests, users, or components

"How long will the instance of the service live?" and "When will a new instance be created?"

There are 3 built-in lifetimes.

Lifetime Description
Transient New instance every time it is requested.
Scoped One instance per HTTP request (or scope).
Singleton One instance for the entire lifetime of the application..

Before diving into service lifetimes, it's very important to understand the core concepts.

  • Dependency Injection (DI)
  • Inversion of Control (IoC)

What is Dependency Injection (DI)?

Dependency Injection is a design pattern used to provide an object’s dependencies from the outside, instead of creating them inside the object itself.

Dependency = Another class or object that your class needs to function.

Injection = Supplying those dependencies externally.

Imagine you're a chef who needs ingredients to cook.

  • Without DI, the chef goes out to the farm, harvests the vegetables, and then cooks.
  • With DI, someone delivers the fresh ingredients to the chef so they can focus on cooking.

💡 DI = Someone gives you what you need. You don’t fetch it yourself.

🛑Without DI (tight coupling)

public class Car
{
    private Engine _engine = new Engine();  // Car creates its own engine

    public void Drive()
    {
        _engine.Start();
    }
}

Problem

  • Hard to test
  • Can’t swap the Engine with the electric engine easily
  • Tight coupling

With DI (loose coupling)

public class Car
{
    private readonly IEngine _engine;

    public Car(IEngine engine)  // Engine is "injected" from outside
    {
        _engine = engine;
    }

    public void Drive()
    {
        _engine.Start();
    }
}

Now we can,

  • Inject any IEngine implementation
  • Test easily (mock the engine)
  • Change logic without touching the Car class

What is Inversion of Control (IoC)?

Inversion of Control is a broader principle. It means that the control of creating and managing objects is inverted — from the class to a container or framework. In simpler terms.

Instead of our code controlling how things are created, the framework (like ASP.NET Core) does it for us.

Example of IoC

We register services in a container.

services.AddScoped<ICarService, CarService>();

Then our controller.

public class HomeController : Controller
{
    private readonly ICarService _carService;

    public HomeController(ICarService carService) // Injected by framework
    {
        _carService = carService;
    }
}

We didn’t create CarService — the IoC container did and gave it to us!

Relationship Between DI and IoC

Concept Meaning Who Manages Creation
IoC Inverting object creation/control Framework/Container
DI A way to implement IoC by injecting dependencies The framework injects what you need

So, Dependency Injection is a technique used to achieve Inversion of Control.

  • IoC: Framework creates and manages the object
  • DI: The Controller gets the service it needs without creating it

Why Are Service Lifetimes Important?

Choosing the correct service lifetime is essential for,

  • Memory management: Avoid unnecessary object creation
  • Performance optimization: Reuse objects when appropriate
  • Thread safety prevents state corruption in multi-user scenarios
  • Correct behavior, especially when services maintain data like a database context
  • Prevents data leaks by isolating instances.
  • Promotes cleaner architecture via proper dependency reuse.

Types of Service Lifetimes in .NET Core

.NET provides three main service lifetimes.

  1. Transient: A new instance is created every time the service is requested.
  2. Scoped: A single instance is created per HTTP request and shared within that request.
  3. Singleton: A single instance is created once and shared across the entire application.

Choosing the correct lifetime ensures better performance, resource management, and application behavior.

Where Are Lifetimes Defined?

Lifetimes are defined when registering services in the Program.cs file.

builder.Services.AddTransient<IMyService, MyService>();   // Transient
builder.Services.AddScoped<IMyService, MyService>();      // Scoped
builder.Services.AddSingleton<IMyService, MyService>();   // Singleton

Service Lifetime Differences in ASP.NET Core

Feature / Criteria Transient Scoped Singleton
Object Creation Every time it is requested Once per HTTP request Once for the entire application
Shared Between Never shared Shared within one request Shared across all requests and components
Reuse Behavior New instance for every injection Same instance reused during one request Same instance reused everywhere
Lifetime Ends Immediately after use End of the HTTP request When the app shuts down
Thread-Safety Required No (short-lived) No (request-specific) Yes (used by multiple threads)
Testing Use Case Best for isolated tests Good for per-request logic Good for app-level state
Best For Lightweight, stateless services Entity Framework DbContext, per-user logic Logging, Caching, Configuration
Common Pitfall High object creation cost Cannot be injected into a Singleton Memory leaks or shared state issues

Best Practices for Choosing Service Lifetimes in .NET

Scenario Recommended Lifetime Real Example Explanation
Stateless utility like a formatter/calculator Transient PriceFormatterService Creates a new instance each time — ideal for small, stateless services like currency formatting or calculations.
Database access, user session services Scoped ApplicationDbContext, UserService Ensures consistency within a single HTTP request — needed for services handling DB transactions or per-user data.
Global logger, configuration, and caching Singleton LoggerService, ConfigProvider Created once and reused across the app — perfect for shared logic like logging or reading global settings.

What Happens If We Choose the Wrong Lifetime?

🔴 1. Scoped Service Injected into Singleton

  • Problem: Scoped services (like DbContext) are created per request. A Singleton lives for the entire app.
  • What Happens: .NET throws a runtime error — InvalidOperationException.
  • Why: A Singleton can't depend on something that changes per request.

🔴 2. Storing Large Data in a Singleton

  • Problem: Singleton services stay alive for the app's entire lifetime.
  • What Happens: If you store large data (e.g., thousands of records), it causes memory leaks or slow performance.
  • Why: Data is never released from memory.

🔴 3. Using Transient for Expensive or Stateful Services

  • Problem: Transient creates a new instance every time.
  • What Happens: Extra memory usage, duplicate work (like database connections or API calls), and inconsistent behavior.
  • Why: Data/state is not shared between calls.

🔴 4. Expecting Transient or Scoped to Act Like Singleton

  • Problem: Assuming the service "remembers" data across requests or injections.
  • What Happens: Data is lost or reset unexpectedly.
  • Why: Scoped lasts only one request, and Transient lasts only one use.

Conclusion

Understanding Service Lifetimes in .NET is essential for building efficient, scalable, and maintainable applications. By mastering Dependency Injection and properly choosing between Transient, Scoped, and Singleton lifetimes, you can control how services behave, improve app performance, and avoid bugs related to memory leaks or shared state.

Always start with Transient unless your scenario calls for Scoped or Singleton. Use Scoped for request-based operations like database access, and use Singleton cautiously for shared logic like configuration or logging, while ensuring thread safety.

With the right service lifetime choices and a solid understanding of DI and IoC, your ASP.NET Core applications will be more testable, modular, and cleanly architected.

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