Myujth

I can use ModuleWithProviders forRoot static methods to register some services with roots modules since those services shared among few components within different modules.

export class SharedModule 
 static forRoot(): ModuleWithProviders 
 return 
 ngModule: SharedModule,
 providers: [ CounterService ]
 
 

But with angular 6 if a services is need to register with root injector simply can be done with @Injector decorator with providedIn metadata set as 'root'.

@Injectable(
 providedIn: 'root'
)
export class UserService 

So what approach should be appropriate means what would be way i should follow?

also does forRoot going to be obsolete??

What happen to Provider Array in Modules??

In angular when a module provides both declaration and providers it would duplicate the provider instances when injecting in child that would cause issues on instances which are probably meant to be singletons.
For this reason Angular provides a way to separate providers out of the module so that same module can be imported into the root module with providers and child modules without providers.

providedIn is the new syntax to create singletons (i.e. providers declared in the root injector).

You can provide it in any other module, but root is the shorthand for AppModule.

It allows you to install dependencies pretty fast and without the need of any additional configuration.

Hence, forRoot won't be obsolete, because it allows one to be able to configure a module through it.

I'd say you should use the providedIn syntax for your providers, and if they need configuration, you can use forRoot.