public void RenderBaseControl(HtmlTextWriter writer)
{
writer.AddAttribute( HtmlTextWriterAttribute.Class, "form-control-group " + this.CssClass );
writer.AddAttribute( HtmlTextWriterAttribute.Id, this.ClientID );
writer.RenderBeginTag( HtmlTextWriterTag.Div );
_tbLowerValue.RenderControl( writer );
writer.Write( "<span class='to'> " + this.RangeLabel + " </span>" );
_tbUpperValue.RenderControl( writer );
writer.RenderEndTag();
}