looking for samples on how to user services.add* in asp.vnext - asp.net-core

Question

I would like to know where can I find samples the explains the differences among services.AddInstance, services.AddScoped, services.AddSingleton and service.AddTransient.
I found some articles that explain the point in a generic way, but I think a source sample is much more clear.

Answer 1

The scope of this questions is rather large, but since it seems you are specifically looking for AddScoped information I narrowed the sample down to scoping inside a web application.
Inside a web application AddScoped will mean pretty much the scope of the request. EntityFramework is using scoping internally, but it doesn't affect the user code in most cases so I'm sticking with the user code as shown below.
If you register a DbContext as a service, and also register a scoped service, for each request you will get a single instance of the scoped service where you resolve the DbContext.
The example code below should make it clearer. In general I would recommend just trying it out the way I'm showing it below to familiarize yourself with the behavior, by stepping through the code in the debugger. Start from an empty web application. Note the code I'm showing is from Beta2 (since in Beta2 we added the [FromServices] attribute which makes it easier to demonstrate, the underlying behavior is the same regardless of version.
startup.cs
public void ConfigureServices(IServiceCollection services)
{
// Add EF services to the services container.
services.AddEntityFramework(Configuration)
.AddSqlServer()
.AddDbContext<UserDbContext>();
services.AddScoped<UserService>();
// Add MVC services to the services container.
services.AddMvc();
}
UserDbContext.cs
public class UserDbContext : DbContext
{
public UserService UserService { get; }
public UserDbContext(UserService userService)
{
_userService = userService;
}
}
HomeController.cs
public class HomeController : Controller
{
private UserDbContext _dbContext;
public HomeController(UserDbContext dbContext)
{
_dbContext = dbContext;
}
public string Index([FromServices]UserDbContext dbContext, [FromServices]UserService userService)
{
// [FromServices] is available start with Beta2, and will resolve the service from DI
// dbContext == _ctrContext
// and of course dbContext.UserService == _ctrContext.UserService;
if (dbContext != _dbContext) throw new InvalidOperationException();
if (dbContext.UserService != _dbContext.UserService) throw new InvalidOperationException();
if (dbContext.UserService != userService) throw new InvalidOperationException();
return "Match";
}
}
Alternatively if you resolve the user service from another service, this time registered as transient the transient service will have a new instance everytime it is resolved, but the scoped service will remain the same within the scope of the request.
Create the new service
public class AnotherUserService
{
public UserService UserService { get; }
public AnotherUserService(UserService userService)
{
UserService = userService;
}
}
Add the following lines to startup.cs
services.AddTransient<AnotherUserService>();
And rewrite the HomeController.cs as follows
public class HomeController : Controller
{
private AnotherUserService _anotherUserService;
public HomeController(AnotherUserService anotherUserService)
{
_anotherUserService = anotherUserService;
}
public string Index([FromServices]AnotherUserService anotherUserService,
[FromServices]UserService userService)
{
// Since another user service is tranient we expect a new instance
if (anotherUserService == _anotherUserService)
throw new InvalidOperationException();
// but the scoped service should remain the same instance
if (anotherUserService.UserService != _anotherUserService.UserService)
throw new InvalidOperationException();
if (anotherUserService.UserService != userService)
throw new InvalidOperationException();
return "Match";
}
}