NG0203: `inject()` must be called from an injection context (a constructor, a factory function or a field initializer)

Description

You see this error when you try to use the inject() function outside of the allowed injection context. The injection context is available during the class creation and initialization.

In practice the inject() calls are allowed in a constructor, a constructor parameter and a field initializer:

@Injectable({providedIn: 'root'})
export class Car {
  radio: Radio|undefined;

  // OK: field initializer
  spareTyre = inject(Tyre);
  
  constructor() {
    // OK: constructor body
    this.radio = inject(Radio);
  }
}

It is also legal to call inject from a provider's factory:

providers: [
  {provide: Car, useFactory: () => {
    // OK: a class factory
    const engine = inject(Engine);
    return new Car(engine);
  }}
]

Calls to the inject() function outside of the class creation context will result in error. Most notably, calls to inject() are disallowed after a class instance was created, in methods (including lifecycle hooks):

@Component({ ... })
export class CarComponent {
  ngOnInit() {
    // ERROR: too late, the component instance was already created
    const engine = inject(Engine);
    engine.start();
  }
}