AjaxControlToolkit.CollapsiblePanelExtender.EnsureValid C# (CSharp) Method

        public override void EnsureValid()
        {
            base.EnsureValid();

            if((ExpandedText != null || CollapsedText != null) && TextLabelID == null)
                throw new ArgumentException("If CollapsedText or ExpandedText is set, TextLabelID must also be set.");
        }