How can I know what kind of service to setup?

Question

I am currently developping a web API, together with the related website. I was about to add in my Startup.cs my new service :

builder.Services.AddScoped<IUserService, UserService>();

But since then, I am wondering how can we know what "type" of service should we add ? There is a bunch of possibilities :

AddScoped
AddSingleton
AddTransient
AddSession
...

And I think this is possibly confusing. So how can we know what type of service makes the most sense for what we want to add (repositories, services, ...) ? What aspects should we take into consideration ?

Also, what impact does it have on our applications ? Does it change anything regarding security, performance or anything else ?

Thanks.

EDIT : here is a better question/explanation

Yes. Different lifetimes introduce different behavior: https://stackoverflow.com/questions/70643677/deciding-the-lifetime-of-dependencies-in-net-core/70653518 Singleton will ensure there can only be a single instance, so would be bad for a stateless API. Scoped is most commonly used since it gives you a new instance per request. — Anthony G., Sep 21 '22 at 19:12
Just FYI `AddSession` is not a lifetime, it's a call which adds [session state support](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/app-state?view=aspnetcore-6.0) to your ASP.NET Core app — Guru Stron, Sep 22 '22 at 19:45

sunriax · Answer 1 · 2022-09-21T19:16:09.983

Take a look at Dependency Injection. It depends on whats the behaviour of your program. The different types describes the lifetime of the services.

Singleton There only will be one instance

Scoped There only will be one instance for the current request

Transient Everytime the instance is used it will be newly generated

If you want to use the same instance during the whole process you can use a singleton (e.g. for session handling). A scoped instance makes sense for things that are necessary during the request proccess.

score 0 · Accepted Answer · answered Sep 22 '22 at 20:02

First of all AddSession is not a lifetime, it's a call which adds session state support to your ASP.NET Core app.

As for scope types - you should read the documentation carefully:

Both articles have quite a lot in common and describe the support of the dependency injection (DI) software design pattern (which is a technique for achieving Inversion of Control (IoC) between classes and their dependencies) which is build into the framework.

This particular section covers the service lifetimes:

Transient
Transient lifetime services are created each time they're requested from the service container. This lifetime works best for lightweight, stateless services. Register transient services with AddTransient.
In apps that process requests, transient services are disposed at the end of the request.

Scoped
For web applications, a scoped lifetime indicates that services are created once per client request (connection). Register scoped services with AddScoped.
In apps that process requests, scoped services are disposed at the end of the request.
When using Entity Framework Core, the AddDbContext extension method registers DbContext types with a scoped lifetime by default.
By default, in the development environment, resolving a service from another service with a longer lifetime throws an exception. For more information, see Scope validation.

Singleton
Singleton lifetime services are created either:

The first time they're requested.

By the developer, when providing an implementation instance directly to the container. This approach is rarely needed.

Every subsequent request of the service implementation from the dependency injection container uses the same instance.

How can I know what kind of service to setup?

2 Answers2