public ICommand SelectionChanged
{
    get
    {
        return new RelayCommand(param => this.ExecuteSelectionChanged(param));
    }
}

private object ExecuteSelectionChanged(object param)
{
    var e = param as SelectionChangedEventArgs;
    var addedItem = e.AddedItems[0];
}

SelectionChanged.Execute(new SelectionChangedEventArgs( null,null, addedList));

SelectionChanged.Execute(new SelectionChangedEventArgs(ComboBox.SelectionChangedEvent, removeList,addList));