Since the SelectedItems property of a RadListBox is of type IList, it is possible to still add items to that list instead of explicitly setting SelectedItems equal to another list.
For example, this will select all names not equal to 'Bobby' by default.
XAML:
<Grid>
<telerik:RadListBox x:Name="ListBox"
SelectionMode="Multiple"/>
</Grid>
Code-Behind:
public partial class MainPage : UserControl
{
public MainPage()
{
InitializeComponent();
IList<string> names = new List<string>();
names.Add("Alexander");
names.Add("Bobby");
names.Add("Chris");
names.Add("Dean");
ListBox.ItemsSource = names;
foreach (var name in names.Where(x => x != "Bobby"))
{
ListBox.SelectedItems.Add(name);
}
}
}