angular - NullInjectorError : No provider for class

Question

我在组件类中将类作为依赖注入(inject)注入(inject)并出现错误

NullInjectorError: No provider for class

这是我的代码:

// Component
import { Formation } from './Models/Formation';

export class FormationComponent {
  constructor(private formation: Formation) { }
}

// Model Class
export class Formation {
}

会有什么问题？

Answer 1

Angular 6+

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class MyService {
    
    constructor() {
    }
}

Angular 5
方法一。在 app.module.ts 中添加类的提供者

@NgModule({
  // ----
  providers: [
    MyService // added class in the providers
  ]
  // ----
})
export class AppModule { }

方法二。
在 component 中添加类的 provider

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.scss'],
  providers: [
    MyService // added class in the providers
  ]
})
export class MyComponent {

  constructor(private service: MyService) {
  }

}

重要提示:请不要忘记将类(class)注释为 @Injectable这样您就可以将其注入(inject)构造函数中，即

import { Injectable } from "@angular/core";

@Injectable() // class annotation as Injectable
export class MyService {

    constructor() {
    }
}