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**Constructor injection** is a way for Angular to **pass dependencies directly into a class's constructor**.

When a component or service is created, Angular looks at what its constructor declares and automatically supplies the needed objects from the injector.

---

### Example:

```ts
import { Component } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-profile',
  template: `{{ userService.userName }}`
})
export class ProfileComponent {
  constructor(public userService: UserService) {}
}
```

Here:

- Angular sees that the constructor needs `UserService`;
- finds it in the injector;
- creates it (if needed) and supplies it as `userService`.

---

**Advantages:**

- dependencies are visible right away, from the constructor's signature;
- easy to test, a fake service can be substituted;
- there is no need to create objects manually.

---

**Summary:**
Constructor injection is the primary way to inject dependencies in Angular: everything a class needs arrives automatically through the constructor's parameters.

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