In order to select all of the ListBox control items you should use the SelectAllCommand command in the Loaded event of the control. the next code snippet represents the described approach:
In the code behind:
sender, RoutedEventArgs e)
If you want to select all the items of RadListBox without using the code behind you should make an attached property and attach it to the control.
On your second question, to deselect all items you can use the SelectedItems.Clear() method. Here is a sample code snippet showing that:
Hope this is helpful.
the Telerik team