Injection of Generic Services in Angular

This is a simple solution:

// service and models
export class User {
  firstName: string
}

export class Admin {
  lastName: string
}

@Injectable()
export class GenericService<T>{
  item: number = Math.random();
  GetAll(): Array<T> {
    let output = [];
    console.log(this.item); // each instance has own value
    return output;
  }
}

Then set your service in module through useFactory:

providers: [
  { provide: 'UserService', useFactory: () => (new GenericService<User>()) },
  { provide: 'AdminService', useFactory: () => (new GenericService<Admin>()) },
],

and inject your service with @Inject decorator:

constructor(
  @Inject('UserService') private userService: GenericService<User>,
  @Inject('AdminService') private adminService: GenericService<Admin>
) { }

Keep in mind it is better to use InjectionToken ( OpaqueToken is deprecated) for your provider token.I have used string just for simplicity.