title: Inject two services in third service angular 4
body: <p><a href="https://i.stack.imgur.com/gtkyY.png" rel="nofollow noreferrer">please find image here</a>I have three services. UserService, storage service, cartservice.
  I am trying to use functions of userservice and storage service in cartservice.
  But I am unable to inject it in constructor. second parameter of service is always undefined.
  code:
  cart service-</p>
  
  <pre><code>import { Injectable } from "@angular/core";
  import { UserService } from "./service";
  import { StorageService } from "./storage.service";
  import { Observable } from "rxjs/Observable";
  import { Observer } from "rxjs/Observer";
  
  @Injectable()
  export class CartService {
  
  public constructor( private storageService: StorageService,
  private _UserService: UserService) {
  console.log('_UserService--' + _UserService);/* Always undefined. It will         work if I switch its position to first in constuctor. but then storageservice will be undefined*/
  </code></pre>
  
  <p>}
  }</p>