To set a default value to mat-select, you have to bind compareWith property, which will compare the values and will set the default value.

Example:

In your HTML:

<mat-form-field class="full-width">
   <mat-select (selectionChange)="dropdownSelectedItem($event)" [(ngModel)]="dropdownSelectedName" [compareWith]="compareFn">
      <mat-option *ngFor="let item of itemList" [value]="item">
      {{item.name}}
      </mat-option>
   </mat-select>
</mat-form-field>

In your TS:

compareFn(obj1: any, obj2: any) {
    return obj1 && obj2 ? obj1.id === obj2.id : obj1 === obj2;
}

• the value of your mat-option is a object reference to a item Object.
• therefore the value that you set on mat-select must be the exact same item object reference.

this means that in your compoenent your code should

this.dropdownSelectedName = [Insert Object Reference Here]

the alternative approach is to change your mat-option to set

<mat-option *ngFor="let item of itemList" [value]="item.name">
  {{item.name}}
<mat-option>

with this change your “myDefaultValue” would match the value of one of the options, instead of being an object reference

Update

Here is a link that shows a working version of setting the value of the select when using object references.

I am using mat-select within Angular Material as below:

<mat-form-field class="full-width">
   <mat-select (selectionChange)="dropdownSelectedItem($event)">
      <mat-option *ngFor="let item of itemList" [value]="item">
      {{item.name}}
      </mat-option>
   </mat-select>
</mat-form-field>

The above dropdown is coming up fine but the dropdown has no default value.

In order to add a default value, I have added value="{{dropdownSelectedName}}" as below:

<mat-form-field class="full-width">
   <mat-select (selectionChange)="dropdownSelectedItem($event)" value="{{dropdownSelectedName}}">
      <mat-option *ngFor="let item of itemList" [value]="item">
      {{item.name}}
      </mat-option>
   </mat-select>
</mat-form-field>

The above change is still not populating the default dropdown value.

I have also tried doing [(value)]="dropdownSelectedName"

When I do console.log(this.dropdownSelectedName) in the component, I can see that it has a string value
mydefaultvalue

ngOnInit() {
    this.subs.sink = this.myService.listAll()
        .subscribe(r => {
            this.AllList = r;
        });
    this.dropdownSelectedName = "mydefaultvalue";
}