|| $jinput->get('filter_month')
|| $jinput->get('filter_corso')

// Get Corso options
$corso              = JFormHelper::loadFieldType('Corso', false);
$corsoOptions       = $corso->getOptions();

// Get Sede options
$sede              = JFormHelper::loadFieldType('Sede', false);
$sedeOptions       = $sede->getOptions();

$filter_corso       = $menuOptions
					? ($this->params->get('filter_corso', 1) * 6)
					: ($iCparams->get('filter_corso', 1) * 6);
$filter_sede       = $menuOptions
					? ($this->params->get('filter_sede', 1) * 6)
					: ($iCparams->get('filter_sede', 1) * 6);

$html_filter_corso      = '<select class="ic-input-small" id="filter_corso" name="filter_corso">
						<option value="">' . JText::_('COM_ICAGENDA_FILTERS_SELECT_CORSO') . '</option>
						' . JHtml::_('select.options', $corsoOptions, 'id', 'value',
						$jinput->get('filter_corso')) . '</select>';

$html_filter_sede      = '<select class="ic-input-small" id="filter_sede" name="filter_sede">
						<option value="">' . JText::_('COM_ICAGENDA_FILTERS_SELECT_SEDE') . '</option>
						' . JHtml::_('select.options', $sedeOptions, 'id', 'value',
						$jinput->get('filter_sede')) . '</select>';

// Filter (dropdown) corso
		$corso = $this->getUserStateFromRequest($this->context . '.filter.corso', 'filter_corso', '', 'string');
		$this->setState('filter.corso', $corso);
		
		// Filter (dropdown) sede
		$sede = $this->getUserStateFromRequest($this->context . '.filter.sede', 'filter_sede', '', 'string');
		$this->setState('filter.sede', $sede);

$search = $app->getUserStateFromRequest($this->context . '.filter.search', 'filter_search');

$id .= ':' . $this->getState('filter.corso');
		$id .= ':' . $this->getState('filter.sede');

<?php
/**
 *------------------------------------------------------------------------------
 *  iCagenda v3 by Jooml!C - Events Management Extension for Joomla! 2.5 / 3.x
 *------------------------------------------------------------------------------
 * @package     com_icagenda
 * @copyright   Copyright (c)2012-2017 Cyril Rezé, Jooml!C - All rights reserved
 *
 * @license     GNU General Public License version 3 or later; see LICENSE.txt
 * @author      Cyril Rezé (Lyr!C)
 * @link        http://www.joomlic.com
 *
 * @version 	3.6.0 2016-07-08
 * @since       3.6.0
 *------------------------------------------------------------------------------
*/

// No direct access to this file
defined('JPATH_BASE') or die;

jimport('joomla.html.html');
jimport('joomla.form.formfield');
jimport('joomla.form.helper');
JFormHelper::loadFieldClass('list');

/**
 * Corso frontend search filter.
 */
class JFormFieldCorso extends JFormFieldList
{
	/**
	 * The form field type.
	 *
	 * @var		string
	 * @since	3.6.0
	 */
	protected $type = 'corso';

	/**
	 * Method to get the field options.
	 *
	 * @return	array	The field option objects.
	 * @since	3.6.0
	 */
	
		/*public function getOptions()
	{
		return '<select id="'.$this->id.'" value="'.$this->value.'">'.
		       '<option value="confermato" >confermato</option>'.
		       '<option value="non confermato" >non confermato</option>'.
		       '</select>';
	}
}*/


public function getOptions()
	{
		$app            = JFactory::getApplication();
		$params         = $app->getParams();

		$filters_mode   = ($app->getMenu()->getActive()->params->get('search_filters') == 1)
						? $params->get('filters_mode', 1)
						: JComponentHelper::getParams('com_icagenda')->get('filters_mode', 1);

		// Initialize variables. 
		$options = array();

		$db    = JFactory::getDbo();
		$query = $db->getQuery(true);

		$query->select('id, slug, value');
		$query->from('#__icagenda_customfields_data AS c');
		//$query->where('state = 1'); AND 'slug = filter_corso'
		$query->where('slug = "filter_corso"');
		$query->group('value');

		// Search in menu filtered list - Cerca nell'elenco dei menu filtrati
		if ($filters_mode == 1)
		{
			$value = $params->get('value', '');
			JArrayHelper::toInteger($value);
			$list_value = implode(',', $value);

			if ($value
				&& count($value) > 0
				&& ! in_array('0', $value))
			{
				$query->where('c.value IN (' . $list_value . ')');
			}
		}

		$query->order('c.value');
		
		
		// Get the options.
		$db->setQuery($query);

		$options = $db->loadObjectList();

		// Check for a database error.
		if ($db->getErrorNum())
		{
			JError::raiseWarning(500, $db->getErrorMsg());
		}

		return $options;
	}
}

<?php
/**
 *------------------------------------------------------------------------------
 *  iCagenda v3 by Jooml!C - Events Management Extension for Joomla! 2.5 / 3.x
 *------------------------------------------------------------------------------
 * @package     com_icagenda
 * @copyright   Copyright (c)2012-2017 Cyril Rezé, Jooml!C - All rights reserved
 *
 * @license     GNU General Public License version 3 or later; see LICENSE.txt
 * @author      Cyril Rezé (Lyr!C)
 * @link        http://www.joomlic.com
 *
 * @version 	3.6.0 2016-07-08
 * @since       3.6.0
 *------------------------------------------------------------------------------
*/

// No direct access to this file
defined('JPATH_BASE') or die;

jimport('joomla.html.html');
jimport('joomla.form.formfield');
jimport('joomla.form.helper');
JFormHelper::loadFieldClass('list');

/**
 * Sede frontend search filter.
 */
class JFormFieldSede extends JFormFieldList
{
	/**
	 * The form field type.
	 *
	 * @var		string
	 * @since	3.6.0
	 */
	protected $type = 'sede';

	/**
	 * Method to get the field options.
	 *
	 * @return	array	The field option objects.
	 * @since	3.6.0
	 */
	
/*		public function getOptions()
	{
		return '<select id="'.$this->id.'" value="'.$this->value.'">'.
		       '<option value="da Parma" >da Parma</option>'.
		       '<option value="da Reggio Emilia" >da Reggio Emilia</option>'.
		       '<option value="da definire" >da definire</option>'.
		       '</select>';
	}
}*/

		public function getOptions()
	{
		$app            = JFactory::getApplication();
		$params         = $app->getParams();

		$filters_mode   = ($app->getMenu()->getActive()->params->get('search_filters') == 1)
						? $params->get('filters_mode', 1)
						: JComponentHelper::getParams('com_icagenda')->get('filters_mode', 1);

		// Initialize variables. 
		$options = array();

		$db    = JFactory::getDbo();
		$query = $db->getQuery(true);

		$query->select('id, slug, value');
		$query->from('#__icagenda_customfields_data AS c');
		//$query->where('state = 1'); AND 'slug = filter_corso'
		$query->where('slug = "filter_sede"');
		$query->group('value');

		// Search in menu filtered list - Cerca nell'elenco dei menu filtrati
		if ($filters_mode == 1)
		{
			$value = $params->get('value', '');
			JArrayHelper::toInteger($value);
			$list_value = implode(',', $value);

			if ($value
				&& count($value) > 0
				&& ! in_array('0', $value))
			{
				$query->where('c.value IN (' . $list_value . ')');
			}
		}

		$query->order('c.value');
		
		
		// Get the options.
		$db->setQuery($query);

		$options = $db->loadObjectList();

		// Check for a database error.
		if ($db->getErrorNum())
		{
			JError::raiseWarning(500, $db->getErrorMsg());
		}

		return $options;
	}
}

'filter_corso'    => '',
							'filter_sede'     => '',

elseif (in_array($fieldname, array('filter_category', 'filter_corso', 'filter_sede', 'filter_month', 'filter_year')))

<field
			name="filter_corso"
			type="hidden"
			rendertype="radio"
			label="COM_ICAGENDA_LIST_SEARCH_FILTERS_CORSO_LBL"
			description=""
			default="1"
			class="btn-group"
			labelclass="control-label"
			>
			<option value="0">JHIDE</option>
			<option value="1">JSHOW</option>
		</field>
		
		<field
			name="filter_sede"
			type="hidden"
			rendertype="radio"
			label="COM_ICAGENDA_LIST_SEARCH_FILTERS_SEDE_LBL"
			description=""
			default="1"
			class="btn-group"
			labelclass="control-label"
			>
			<option value="0">JHIDE</option>
			<option value="1">JSHOW</option>
		</field>

da 			default="filter_search,filter_from,filter_to,filter_category,filter_month,filter_year"
a 			default="filter_search,filter_from,filter_to,filter_category,filter_corso,filter_sede,filter_month,filter_year"

da	default="filter_search,filter_from,filter_to,filter_category,filter_month,filter_year"
a	default="filter_search,filter_from,filter_to,filter_category,filter_corso,filter_sede,filter_month,filter_year"

<field
				name="filter_corso"
				type="hidden"
				rendertype="radio"
				label="COM_ICAGENDA_LIST_SEARCH_FILTERS_CORSO_LBL"
				description=""
				default=""
				class="btn-group hidden-j25"
				labelclass="control-label hidden-j25"
				useglobal="true"
				showon="search_filters:1"
				>
				<option value="0">JHIDE</option>
				<option value="1">JSHOW</option>
			</field>
			
			<field
				name="filter_sede"
				type="hidden"
				rendertype="radio"
				label="COM_ICAGENDA_LIST_SEARCH_FILTERS_SEDE_LBL"
				description=""
				default=""
				class="btn-group hidden-j25"
				labelclass="control-label hidden-j25"
				useglobal="true"
				showon="search_filters:1"
				>
				<option value="0">JHIDE</option>
				<option value="1">JSHOW</option>
			</field>