Angular9 Multiple Checkbox Delete - asp.net

Question

I Use Table with selection On Angular Material For Multiple Delete I'm Newbie On angular
Now My Project Use .net C# Angular And Basic CRUD
I Want Return Object or Array For Use on My Controller But On my Code It Return "0"
This My Code
Html
<button (click)="DeleteData()" value="Delete">DELETE</button>
<div class="mat-elevation-z8">
<table mat-table [dataSource]="dataSource">
<ng-container matColumnDef="select">
<th style="width: 100px;" mat-header-cell *matHeaderCellDef>
<mat-checkbox (change)="$event ? masterToggle() : null"
[checked]="selection.hasValue() && isAllSelected()"
[indeterminate]="selection.hasValue() && !isAllSelected()"></mat-checkbox>
</th>
<td mat-cell *matCellDef="let row">
<mat-checkbox (click)="$event.stopPropagation()" (change)="$event ? selection.toggle(row) : null"
[checked]="selection.isSelected(row)" [aria-label]="checkboxLabel(row)"></mat-checkbox>
</td>
</ng-container>
<ng-container matColumnDef="Name">
<th mat-header-cell *matHeaderCellDef> Name </th>
<td mat-cell *matCellDef="let element">{{element.Name}} </td>
</ng-container>
<tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
<tr mat-row *matRowDef="let row; columns: displayedColumns;"(click)="selection.toggle(row)"></tr>
</table>
</div>
Component
refreshList() {
this.service.getdetail()
.subscribe(data => {
this.dataSource = new MatTableDataSource(data);
})
}
isAllSelected() {
const numSelected = this.selection.selected.length;
const numRows = !!this.dataSource && this.dataSource.data.length;
return numSelected === numRows;
}
masterToggle() {
this.isAllSelected() ? this.selection.clear() : this.dataSource.data.forEach(r => this.selection.select(r));
}
checkboxLabel(row: detail): string {
if (!row) {
return `${this.isAllSelected() ? 'select' : 'deselect'} all`;
}
return `${this.selection.isSelected(row) ? 'deselect' : 'select'} row ${row.Id + 1}`;
}
DeleteData() {
const numSelected = this.selection.selected;
this.service.deleteData(numSelected).subscribe(result => {
alert(result);
this.refreshList();
})
}
Services
deleteData(num: detail[]): Observable< string > {
return this.http.post< string>(this.rootURL + '/WebAPI/DeleteRecord', num);
}
export class detail {
Id: number;
Name: string;
}
This My Result Picture