Understanding Service Lifetimes in ASP.NET Core

In ASP.NET Core, the Dependency Injection (DI) container is responsible for managing the lifecycle of services and dependencies throughout the application. When a service is registered with the DI container, you also define its "lifetime," which determines how long an instance of that service will be reused before the DI container creates a new one. ASP.NET Core provides three common lifetimes: Transient, Scoped, and Singleton.

1. Transient

A Transient service is created each time it is requested from the DI container. This means a new instance of the service is created every time it's injected into a class or method. Transient services are best suited for lightweight, stateless objects that don't need to maintain any shared state between requests or instances.

Example:

public interface ITransientService
{
    void Execute();
}

public class TransientService : ITransientService
{
    public void Execute() => Console.WriteLine("Transient service executed");
}

services.AddTransient<ITransientService, TransientService>();

In this case, every time ITransientService is injected into a class, a new instance of TransientService will be created.

2. Scoped

A Scoped service is created once per request (or scope). This means that within the context of a single HTTP request, every time the service is requested, the same instance is used. Once the request is complete, the instance is discarded. This makes scoped services ideal for scenarios where you need to maintain a shared state across operations within a single request but don't want to carry that state beyond the request's lifecycle.

Example:

public interface IScopedService
{
    void Execute();
}

public class ScopedService : IScopedService
{
    public void Execute() => Console.WriteLine("Scoped service executed");
}

services.AddScoped<IScopedService, ScopedService>();

In this case, ScopedService will maintain the same instance throughout the lifetime of a single HTTP request, but a new instance will be created for the next request.

3. Singleton

A Singleton service is created only once and shared across the entire application. The same instance of the service is used for every request and every injection. Singleton services are best suited for heavy, stateful objects that are expensive to create and need to be reused throughout the application. However, be careful with shared state in singletons, as concurrent requests could lead to unexpected behavior if the service is not properly designed to handle concurrency.

Example:

public interface ISingletonService
{
    void Execute();
}

public class SingletonService : ISingletonService
{
    public void Execute() => Console.WriteLine("Singleton service executed");
}

services.AddSingleton<ISingletonService, SingletonService>();

In this case, only one instance of SingletonService will be created for the entire lifetime of the application. Every time it's requested, the same instance is returned.

Key Differences:

• Transient: A new instance is created every time the service is requested.
• Scoped: A new instance is created for each HTTP request, and the same instance is used throughout that request.
• Singleton: A single instance is created and shared across the entire application.

When to Use Each:

• Transient services are ideal for lightweight and stateless services where the cost of creating a new instance is low.
• Scoped services are great when you need to maintain some shared state across the operations within a single request, such as a database context.
• Singleton services are useful when the service is heavy to create or when you need to ensure that only one instance exists and is shared across the entire application.

Understanding these lifetimes helps you manage resources efficiently, control the scope of your services, and prevent unwanted side effects caused by incorrectly shared state in your web applications.