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**Module injector** is an injector created **for a specific module or a section of the application**.
It lives inside the application but **below the root injector** in the hierarchy.

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### How it works:

- if Angular does not find a dependency in the module injector,
it goes up the chain, to the root injector;
- if a dependency is registered only in the module,
it is available **only within that module**.

---

### Example:

```ts
@NgModule({
  providers: [UserService]
})
export class AdminModule {}
```

Here `UserService` is registered in the **injector** of `AdminModule`.
It will only be available to the components, directives, and services of that module.

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### Why it is needed:

- to **isolate services** within a specific part of the application;
- to **avoid conflicts** between identical dependencies in different modules;
- to allow **separate instances of services** for each section.

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**Summary:**
A module injector is a local dependency container
that manages services only within its own module and interacts with the root injector through the hierarchy.

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