NG0200: Circular dependency in DI detected，Why and How to do?

I want inject ParentCompoent to ChildComponent，

But have a error is：

Error: NG0200: Circular dependency in DI detected for ParentComponent

parent Component：

@Component({
selector: 'cy-parent',
templateUrl: './parent.component.html',
styleUrls: ['./parent.component.scss'],
providers: [
{provide: ParentComponent, useExisting: forwardRef(() => ParentComponent)}
]
})

export class ParentComponent {
constructor() { }
}


child component：

@Component({
selector: 'cy-child',
templateUrl: './child.component.html',
styleUrls: ['./child.component.scss'],

})
export class ChildComponent implements OnInit {
constructor(
@SkipSelf() @Optional() public _parentComp?: ParentComponent
) {

}
}

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