SelectedInvitees.Clear(); foreach (EventInviteViewModel invitee in Invitees.SourceCollection) { SelectedInvitees.Add(invitee); } Also If you want to set other properties based on the collection, e.g. the count of selected items or a button that is enabled/disabled on the count of selected items, you need to fire their property changed events when the SelectedItems collection changes - either from