The following shows use with the @Optional decorator, and allows for a null result.

class OtherService {}
        class HostService {}

        @Directive({
          selector: 'child-directive',
          standalone: false,
        })
        class ChildDirective {
          logs: string[] = [];

          constructor(@Optional() @Host() os: OtherService, @Optional() @Host() hs: HostService) {
            // os is null: true
            this.logs.push(`os is null: ${os === null}`);
            // hs is an instance of HostService: true
            this.logs.push(`hs is an instance of HostService: ${hs instanceof HostService}`);
          }
        }

        @Component({
          selector: 'parent-cmp',
          viewProviders: [HostService],
          template: '<child-directive></child-directive>',
          standalone: false,
        })
        class ParentCmp {}

        @Component({
          selector: 'app',
          viewProviders: [OtherService],
          template: '<parent-cmp></parent-cmp>',
          standalone: false,
        })
        class App {}

For an extended example, see "Dependency Injection Guide".