Windows 8 GridView multi selected items

I am developing an application for Windows 8, I would like to select multiple items in a GridView (by c# code), I tried this: 1st for (int i = 0; i <= 2; i++) { this.ItemGridView.SelectedIndex = i; } //in this way is only selects the third element 2nd this.ItemGridView.SelectedItem = listPeople; //in this way does not select anything 3rd foreach (Persona persona in listaPersone) { this.ItemGridVie