private void FixupLevels(object sender, NotifyCollectionChangedEventArgs e)
{
if (e.NewItems != null)
{
foreach (Level item in e.NewItems)
{
item.Course = this;
}
}
if (e.OldItems != null)
{
foreach (Level item in e.OldItems)
{
if (ReferenceEquals(item.Course, this))
{
item.Course = null;
}
}
}
}