What is the benefit of register service in app.module.ts vs individual component in angular4?

Question

For example if I register a service in app.module.ts I have to do the following to make a service available in a component

app.module.ts

import { AppComponent } from './app.component';
import { MyComponent } from './mycomponent/mycomponent.component';
import { MyService } from './services/myservice.service';


@NgModule({
  declarations: [
    AppComponent,
    MyComponent
  ],
  imports: [
  ],
  providers: [MyService],
  bootstrap: [AppComponent]
})

mycomponent.component.ts

import { Component, OnInit } from '@angular/core';
import { MyService } from '../services/myservice.service';

@Component({
    selector: 'my-selector',
    templateUrl: './mycomponent.component.html',
    styleUrls: ['./my-component.component.scss']
})
export class MyComponent implements OnInit {

    constructor(private _myService: MyService) { }

    ngOnInit() {
        this._myService.test()
    }

}

But I could also just register in the component itself to use it without having to add anything to app.module.ts

import { Component, OnInit } from '@angular/core';
import { MyService } from '../services/myservice.service';

@Component({
    selector: 'my-selector',
    templateUrl: './mycomponent.component.html',
    styleUrls: ['./my-component.component.scss'],
    providers: [MyService]
})
export class MyComponent implements OnInit {

    constructor(private _myService: MyService) { }

    ngOnInit() {
        this._myService.test()
    }

}

I am curious, what is the difference for each way?

Answer 1

When you add your service in the root providers of your application, all components will inject the same instance of the service. So only one instance will be created during the application. In this case you can keep state in your service.

When you add your service in the providers of your Component, every time a new instance of that component will be created, new instance of service also will be created. In this case you can't keep state in this type of service,

Depending on your logic provide where you need, but it's common case to provide all services in the root module.