功能模块提供服务

Question

我似乎明白，如果我在FeatureModule中提供服务然后在我的AppModule（根模块）中导入该模块，则功能模块中列出的所有提供程序都可以在应用程序范围内使用

然而，这段代码似乎不起作用。谁能帮我理解我做错了什么？

功能

@NgModule({
  imports: [ CommonModule ],
  declarations: [],
  providers: [
    DatetimeService, RandomService
  ]
})
export class FeaturesModule { }

根模块

@NgModule({
  declarations: [ AppComponent, ChildComponent ],
  imports: [
    BrowserModule,
    FormsModule,
    FeaturesModule
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

当我尝试在RandomService中注入AppComponent时，它会失败地失败

Error: StaticInjectorError[RandomService]: 
  StaticInjectorError[RandomService]: 
    NullInjectorError: No provider for RandomService!
    at _NullInjector.get (core.js:923)
    at resolveToken (core.js:1211)
    at tryResolveToken (core.js:1153)
    at StaticInjector.get (core.js:1024)
    at resolveToken (core.js:1211)
    at tryResolveToken (core.js:1153)
    at StaticInjector.get (core.js:1024)
    at resolveNgModuleDep (core.js:10584)
    at NgModuleRef_.get (core.js:11805)
    at resolveDep (core.js:12301)

我正在使用最新的Angular 5。