从onSubmit值获取undefined

Question

我无法检索表单值，它以某种方式让我不确定我做错了什么？

我的HTML：

  <form enctype="multipart/form-data" #studentUploadForm="ngForm" (ngSubmit)="onSubmit(studentUploadForm)">
    <label for="exampleInputFile">Examen:</label>

    <div class="form-group">
      <select [(ngModel)]="examen_id" name="examenId" class="form-control">
        <option [ngValue]="examen.id" *ngFor="let examen of examensStudent">{{examen.naam}}</option>
      </select>
    </div>

    <label for="exampleInputFile">Bestand:</label>

    <input type="file" class="form-control-file" id="exampleInputFile" aria-describedby="fileHelp">
    <small id="fileHelp" class="form-text text-muted">Upload hier de examen die je wilt inleveren.</small>

    <button type="submit" class="pxl-knop">Upload</button>
  </form>

我的TS：

export class StudentUploadComponent implements OnInit {
  @Input() examensStudent: Examen[] = [];
  examen_id = 1;

  constructor(private serv: ExamService) { }

  onSubmit(form) {
    console.log(form.examen_id); //This is returning undefined
  }

  ngOnInit() {
    this.serv.getExams().subscribe(data => this.examensStudent = data);
  }

}