角度2“无法读取未定义的属性”dataService“`

我正在使用Angular 2,并试图在方法内部调用它,但出现了一个错误: Cannot read property 'dataService' of undefined

passwordMatchValidator(g: FormGroup) {
                        console.log(this.dataService);
                        return g.get('password').value === g.get('repeat').value ? null : {'mismatch': true};
        }

    import { DataService } from '../../services/data.service';

    @Component({
            selector: 'app-register',
            templateUrl: './register.component.html',
            styles: []
    })
    export class RegisterComponent{

    constructor(private fb: FormBuilder,
                    private locationService: LocationService,
                    private dataService: DataService){}

buildForm(): void {    

this.step1Form = this.fb.group({
                username: new FormControl('', {
                        validators: [Validators.required,
                        Validators.minLength(4)],
                }),
                email: new FormControl('', {
                        validators: [Validators.required,
                        Validators.pattern("^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$")],
                }),
                password: new FormControl('', {
                        validators: [Validators.required,
                        Validators.minLength(8)],
                }),
                repeat: new FormControl('', {
                        validators: [Validators.required,
                        Validators.minLength(8)],
                }),
                }, { validator: this.passwordMatchValidator });

}

    passwordMatchValidator(g: FormGroup) {
                            console.log(this.dataService);
                            return g.get('password').value === g.get('repeat').value ? null : {'mismatch': true};
            }

    }