Angular 6 mat-form-field with mat-select不选择任何值

我是Angular 6的新手，我正在开展一个项目，我正在使用具有反应形式的材料设计，使用表单控件 . 所以我面临的问题是除了mat-select之外所有其他字段都正常工作，它正确显示下拉内容，但是当我选择它时，所选值不会反映在表单控件中 . mat-select数据来自后端

这是我的HTML代码：

<mat-form-field class="example-full-width">
      <mat-select placeholder="Source City" formControlName="sourceCityControl" required [errorStateMatcher]="matcher">
        <mat-option *ngFor="let city of cities" [value]="city">
          {{city.name}}
        </mat-option>
      </mat-select>
      <mat-error>
        Source City <strong> Required </strong>
      </mat-error>
    </mat-form-field>

    <br> <br>

    <mat-form-field class="example-full-width">
      <mat-select placeholder="Destination City" formControlName="destinationCityControl" required [errorStateMatcher]="matcher">
        <mat-option *ngFor="let city of cities" [value]="city">
          {{city.name}}
        </mat-option>
      </mat-select>
      <mat-error>
        Destination City <strong> Required </strong>
      </mat-error>

    </mat-form-field> 
    <!-- other form elements  -->
    <!-- to check form elements -->
    {{profileForm.value | json}}

这是我的TypeScript文件代码：

public cities = [];
minDate = new Date();
nextdate = this.minDate.getUTCDate();
nextmonth = this.minDate.getUTCMonth();
nextyear = this.minDate.getFullYear();
maxDate = new Date(this.nextyear, this.nextmonth + 1, this.nextdate);
host: Oldoffer;
offerModel: any;
matcher = new MyErrorStateMatcher();
constructor(private fb: FormBuilder, private _offerService: 
OfferRideService, private router: Router, private editService: EditService) 
{ }
test1: string;
test2: Date;
test3: Date;
profileForm: FormGroup;
ngOnInit() {
console.log(this.nextdate);
this._offerService.getCities().subscribe(data => this.cities = data);
this.editService.getHost().subscribe((data) => {
  this.host = data;
  this.test1 = this.host.date.toString();
  this.test2 = new Date(this.test1.slice(0, 19) + 'Z');
  this.test3 = new Date(this.test2.getFullYear(), this.test2.getUTCMonth(), 
  this.test2.getUTCDate())
  this.profileForm = new FormGroup({
    sourceCityControl: new FormControl('', Validators.required),
    destinationCityControl: new FormControl('', Validators.required),
    dateControl: new FormControl(this.test3, Validators.required),
    seatsControl: new FormControl(this.host.noOfSeats, [Validators.required, 
    Validators.min(1), Validators.max(4)]),
    amountControl: new FormControl(this.host.amount, [Validators.required, 
    Validators.min(0), Validators.max(2000)]),
    acControl: new FormControl(this.host.preference.ac),
    musicControl: new FormControl(this.host.preference.music),
    smokingControl: new FormControl(this.host.preference.smoking),
    petsControl: new FormControl(this.host.preference.pets)
  });
})
}//other code omitted

任何帮助和建议？谢谢 ...