Make sure a service is instantiated

Services are instantiated when you provide them.

What you are doing in the constructor is injecting an existing service instance into your class so that it can be used.

There are three ways to instantiate or provide a service:

One is in the app module.

@NgModule ({
    providers: [ myservice1, myservice2],
})

This instantiates the service and makes it available throughout the application.

You can also provide it in a feature module, or shared module, and it accomplishes the same thing. If you provide it in the app module, then provide the same service in a feature module that is lazy loaded, it instantiates a second one that becomes globally available. The first one disappears into the ether.

If you want to instantiate a service for a specific component, and possibly have multiple instantiations one for each component, then you provide it in the component metadata.

@Component ({
     providers: [myservice]
})

Then you would inject it using the constructor.

As mentioned in various comments, one option is simply to directly instantiate such services, this would look like

// app.module.ts

@NgModule({
  providers: [
    { provide: AppService1, useValue: new AppService1() },
    { provide: AppService2, useValue: new AppService2() },   
    { provide: AppService2, useValue: new AppService3() }
  ]
}) export class AppModule {}

You might be tempted to avoid direct instantiation because it goes against the everything is handled by the Injector mindset, but it doesn't break DI or testability for a number of reasons.

One reason is that the use of ES Modules, combined with the use of a configurable loader, and the expressiveness of TypeScript's structurally typed nature, allow even these sorts of dependencies to be swapped for test doubles at runtime by a leveraging a loader like SystemJS.

That said, if you find yourself doing this very frequently, you may need to re-evaluate your app structure, but in general there are plenty of use cases for which this solution is the simplest. Also, it can break cycles in the injector.

By break cycles in the injector, I mean that it is possible to capture an instance of a required service that would otherwise need to be injected in the constructor of another service by simply referencing its value in the expression specified for useValue. This technique is even more useful with useFactory. Regardless, this is fairly uncommon but can be a useful workaround.

Another pattern of making sure a service gets instantiated depending on your preference could be to inject it into a module constructor.

This way the service gets instantiated together with the module.

So, instead of creating a brand new service just to inject another services into it as you described in your question, you could just do something like this:

@NgModule ({
    ...
})
export class SomeModule {
    // Services are not used, just to make sure they're instantiated
    constructor(
        appService1: AppService1,     
        appService2: AppService2,
        appService3: AppService3) {
    }
}

This approach has at least 2 benefits as compared to the useValue: new AppService1() approach.

The first and the obvious one, if AppService1 has dependencies, they would be resolved automatically by the Angular's DI.

The second, often services that are not actually referenced anywhere from your app are some king of global configuration services. So, you can combine such service instantiation with a global configuration in a single source file. Here is an example. In this case, this is NgbDatepickerConfig service:

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

import {
    NgbDateAdapter, NgbDateNativeUTCAdapter, NgbDateParserFormatter, NgbDatepickerConfig, NgbDatepickerModule,
    NgbDropdownModule, NgbTabsetModule
} from "@ng-bootstrap/ng-bootstrap";

import { UsDateParserFormatter } from "./us-date-parser-formatter";

@NgModule({
    exports: [
        NgbDatepickerModule,
        NgbDropdownModule,
        NgbTabsetModule
    ],
    providers: [
        { provide: NgbDateAdapter, useClass: NgbDateNativeUTCAdapter },
        { provide: NgbDateParserFormatter, useClass: UsDateParserFormatter }
    ]
})
export class NgbImportsModule {
    public constructor(datepickerConfigService: NgbDatepickerConfig) {
        datepickerConfigService.minDate = { year: 1900, month: 1, day: 1 };
        datepickerConfigService.maxDate = { year: 2099, month: 12, day: 31 };
    }
}

In this example, NgbImportsModule was initially introduced just to re-export required modules from NGB to the rest of my app. But as my app features were building up, NgbImportsModule turned into a single place where certain parts of NGB are configured conveniently in one place.