Understanding Dependency Injection and Service Lifetimes in ASP.NET Core

A clear explanation of Dependency Injection in ASP.NET Core and how to choose the right service lifetime — Transient, Scoped, or Singleton — for clean, scalable applications.

In ASP.NET Core, Dependency Injection (DI) is one of the core design patterns that shape how modern .NET applications are built. It helps keep your code loosely coupled, easier to test, and more maintainable over time. But to use it effectively, you need to understand how services are created and how long they live — what we call service lifetimes.

What is Dependency Injection?

Dependency Injection is a technique that allows you to remove the responsibility of creating dependencies from your classes. Instead of instantiating objects directly with the new keyword, you ask the framework to provide them. This is done through a container that manages the creation, configuration, and lifetime of your services.

For example, if your controller depends on a repository, the DI container ensures that the repository is automatically provided when the controller is created. This makes your code more flexible and easier to test since dependencies can be replaced with mocks or alternative implementations.

At the core of this system is the service container, which controls how and when instances of your services are created. And that’s where service lifetimes come in.

// Programs.cs

services.AddSingleton<IUserService, UserService>(); // This is the dependency injection

// HomeController.cs

public class HomeController
{
    private readonly UserService _service;

    public HomeController(UserService service)
    {
        _service = service; // You retrieve the service by dependency injection
    }
}

Understanding Service Lifetimes

When you register a service in the DI container, you specify its lifetime — how long the service instance should live and how it is shared across the application. ASP.NET Core defines three main lifetimes:

• Transient — A new instance is created each time it’s requested.
• Scoped — A single instance is created per HTTP request and shared within that request.
• Singleton — A single instance is created and shared for the entire lifetime of the application.

Each of these lifetimes has a specific purpose, and choosing the right one has a direct impact on your application’s behavior, performance, and memory usage.

Transient Services

A Transient service is created every time it’s requested. If your service is injected into multiple components, each of them will receive a different instance.

This makes transient services ideal for stateless and lightweight operations — things like helper classes, mappers, or formatters that don’t need to maintain any state between calls. Since a new instance is created every time, you avoid unwanted side effects, but at the cost of slightly higher object creation overhead.

Transient services should be simple and fast to instantiate. Avoid using them for expensive operations or stateful components.

// Programs.cs - Example of transient service

services.AddTransient<IEmailFormatter, EmailFormatter>();

Scoped Services

A Scoped service is created once per HTTP request and shared across all components that participate in handling that request. That means controllers, middlewares, or other services used during the same request all share the same instance.

This lifetime is particularly useful when you need consistency across a request — for example, when working with Entity Framework’s DbContext. Using a scoped service ensures that the same database context is used throughout the request, preventing issues like multiple concurrent database connections or inconsistent data states.

If you accidentally inject a scoped service into a singleton, you’ll run into runtime errors. This is because the singleton lives longer than the scoped service, and the framework prevents that mismatch to avoid invalid references.

// Programs.cs - Example of scoped service

services.AddScoped<MyDbContext>();

Singleton Services

A Singleton service is created once and shared across the entire application. Every consumer that requests it will receive the same instance, regardless of the request or the user.

Singletons are ideal for services that hold shared state or perform global operations — for example, caching, configuration management, or logging. Because they persist for the lifetime of the application, they’re efficient to use when you want to avoid creating the same object repeatedly.

However, singleton services require extra care when managing state. Since they are shared across all requests and threads, they must be thread-safe. Any mutable data stored in a singleton can cause concurrency issues if not handled properly.

// Programs.cs - Example of scoped service

services.AddSingleton<IAppConfiguration, AppConfiguration>();

Choosing the Right Lifetime

Selecting the correct service lifetime is less about rules and more about understanding how your service behaves.

• Use Transient for stateless operations that don’t depend on shared data.
• Use Scoped for operations that require consistency throughout a request, like database contexts.
• Use Singleton for shared resources that are expensive to create or represent global state.

In general, you should start with Scoped as the default lifetime for application services, and only use Singleton or Transient when there’s a clear reason to do so. This balance keeps your app efficient without overcomplicating its object management.

Common Pitfalls

Misusing service lifetimes can cause subtle bugs:

• Injecting a scoped service into a singleton will cause runtime errors.
• Storing state in a singleton that changes per request can lead to concurrency issues.
• Overusing transient services can lead to unnecessary object creation and reduced performance.

By understanding how each lifetime behaves, you can avoid these traps and design systems that are both scalable and predictable.

Final Thoughts

Dependency Injection in ASP.NET Core is more than a pattern — it’s a foundation. Once you understand how the container manages object creation and lifetimes, you gain precise control over how your application behaves under load and how it scales.

Choosing the right lifetime for your services helps you write cleaner, more efficient code that is easy to reason about and maintain. The framework gives you the flexibility; your job is to use it wisely.