/**
  * @return string
  */
 function ToString()
 {
     $retval = '';
     $lineStr = '--';
     if ($this->List->Count() > 0) {
         $bound = $this->_parent->GetBoundary();
         foreach ($this->List->Instance() as $mimePart) {
             $retval .= $lineStr . $bound . CRLF;
             $retval .= $mimePart->ToString() . CRLF;
         }
         $retval .= $lineStr . $this->_parent->GetBoundary() . $lineStr . CRLF;
     }
     return $retval;
 }