Cannot read property 'isXXXXX' of undefine

问题:

I am trying to use a shared service on one of my components. When I am using with app root component then it is working fine. When I am trying to use on another module/dashboard then I am getting an error.

shared/authorize.service.ts

@Injectable()
export class AuthorizationService {
  isDoingSomething() {
    return "Some Boolean value";
  }
}

dashboard/header/header.component.ts

@Component({
  selector: 'app-header',
  templateUrl: './header.component.html',
  styleUrls: ['./header.component.css']
})
export class HeaderComponent {
 constructor(private _auth: AuthorizeService,
          private _router: Router) { }
}

dashboard/header/header.component.html

<mat-toolbar color="primary" role="heading">
  <div fxFlex fxLayout fxLayoutAlign="flex-end">
    <ul fxLayout fxLayoutGap="10px" class="navigation-items">
      <li routerLinkActive="active" [routerLinkActiveOptions]="{exact: true}">
        <a routerLink="/home">Home</a>
      </li>
      <li routerLinkActive="active">
        <a *ngIf="_auth.isDoingSomething()" routerLink="/docall">Rest Call</a>
      </li>
    </ul>
  </div>
</mat-toolbar>

**HeaderComponent.html:6 ERROR TypeError:** Cannot read property 'isDoingSomething' of undefined
     at Object.eval [as updateDirectives] (HeaderComponent.html:6)
      at Object.debugUpdateDirectives [as updateDirectives] (core.js:23911)
      at checkAndUpdateView (core.js:23307)

回答1:

I think you are using lazy-loading in your app if it is, you must create a shared module and add your service in its providers, then import it in each module you want to use it.

@NgModule({
  imports: [],
  declarations: [],
  exports:[],
  providers: [
     YourService
  ]
})

 export class SharedModule {
}