如何不在mat-select中显示选定的值

<mat-form-field>
  <mat-select placeholder="请选择">
    <mat-option value="option1">选项1</mat-option>
    <mat-option value="option2">选项2</mat-option>
    <mat-option value="option3">选项3</mat-option>
  </mat-select>
</mat-form-field>

<mat-form-field>
  <mat-select [displayWith]="displayFn">
    <mat-option value="option1">选项1</mat-option>
    <mat-option value="option2">选项2</mat-option>
    <mat-option value="option3">选项3</mat-option>
  </mat-select>
</mat-form-field>

displayFn(value: string): string {
  // 返回空字符串来隐藏选定的值
  return '';
}

<mat-form-field>
  <mat-select>
    <mat-option value="option1">选项1</mat-option>
    <mat-option value="option2">选项2</mat-option>
    <mat-option value="option3">选项3</mat-option>
  </mat-select>
</mat-form-field>

.mat-select-value {
  display: none;
}