[CheckedDropDownList] Load-on-Demand

private void LoadOnDemandCompleted( object sender, GetDropDownMenuItemsPagedCompletedEventArgs e )
{
	//RadCheckedDropDownListOnDemand - my class that inherits RadCheckedDropDownList
	if ( e.Error != null || e.Result == null || (int)e.UserState != this.RadCheckedDropDownListOnDemand.RequestId )
	{
		//WS call errored out, nothing was returned, or the request number is not matching
		return;
	}

	if ( !( this.RadCheckedDropDownListOnDemand.DataSource is List<RadCheckedListDataItem> mainDataSource ) )
		return;

	//Properties explanation:
	//Value == string to display
	//Id == id in database
	//ChoiceOrder - order in which the RadCheckedListDataItem should appear -> lower ChoiceOrder means it is higher on the list

	var itemsFiltered = e.Result.Select( re => new RadCheckedListDataItem( re.Value )
	{
		Value = re.Id,
		Tag = re.ChoiceOrder,
	} )
	.Where( item => !mainDataSource.Any( mainDataSourceItem => (int)item.Value == (int)mainDataSourceItem.Value ) )
	.ToList();

	//don not want to add items that are already existing in 
	if ( itemsFiltered.Count > 0 )
	{					
		mainDataSource.AddRange( itemsFiltered );

		//want to refresh item list for CheckedDropDownListElement
		if ( !( this.RadCheckedDropDownListOnDemand.CheckedDropDownListElement.DataSource is List<RadCheckedListDataItem> listElementDataSource ) )
			return;

		listElementDataSource.AddRange( itemsFiltered );

		//this does not show the newly added items
		this.RadCheckedDropDownListOnDemand.CheckedDropDownListElement.ShowPopup();				
	}
}