protected override void OnItemRemoving(ColumnBase item)
{
Debug.Assert(item != null);
item.PropertyChanged -= new PropertyChangedEventHandler(this.OnColumnPropertyChanged);
item.RealizedContainersRequested -= new RealizedContainersRequestedEventHandler(this.OnColumnRealizedContainersRequested);
item.ActualWidthChanged -= new ColumnActualWidthChangedEventHandler(this.OnColumnActualWidthChanged);
item.DistinctValuesRequested -= new DistinctValuesRequestedEventHandler(this.OnColumnDistinctValuesRequested);
item.DetachFromContainingCollection();
base.OnItemRemoving(item);
}