NG0203.md

@name inject() must be called from an injection context @category runtime @shortDescription inject() must be called from an injection context such as a constructor, a factory function, a field initializer, or a function used with EnvironmentInjector#runInContext.

@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. It is also available to functions used with EnvironmentInjector#runInContext.

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 or runInContext 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();
  }
}

@debugging

Work backwards from the stack trace of the error to identify a place where the disallowed call to inject() is located.

To fix the error move the inject() call to an allowed place (usually a class constructor or a field initializer).

@reviewed 2022-05-27