How to implement select all in angular material drop down angular 5

Use click event try this

<mat-form-field>
  <mat-select placeholder="Toppings" [formControl]="toppings" multiple>
    <mat-option [value]="1" (click)="selectAll(ev)"   
    #ev
     >SelectAll</mat-option>
    <mat-option *ngFor="let topping of toppingList" [value]="topping">{{topping}}</mat-option>
  </mat-select>
</mat-form-field>

}

Example:https://stackblitz.com/edit/angular-czmxfp