private string ControlHtml()
{
if (_append.Count == 0 && _prepend.Count == 0) return _control.ToHtmlString();
return string.Format("<div class=\"{0}\">{1}{2}{3}</div>",
string.Format("{0} {1}", _prepend.Count > 0 ? "input-prepend" : "", _append.Count > 0 ? "input-append" : ""),
string.Join("", _prepend.Select(x => x.ToHtmlString())),
_control,
string.Join("", _append.Select(x => x.ToHtmlString()))
);
}