WPF MVVM ComboBox SelectedItem or SelectedValue not working

Question

Update

After a bit of investigating. What seems to be the issue is that the SelectedValue/SelectedItem is occurring before the Item source is finis

Answer 1

Use Loaded event:

private void cmb_Loaded(object sender, RoutedEventArgs e) {
    if (cmb.Items.Count > 0) cmb.SelectedIndex = 0;          
}

It works for me.