问题缘起：
$find('ctl00_ContentPlaceHolder1_MyAccordion_AccordionExtender')
这里找的是什么呢？一开始我以为是找控件，实际上找的是一个AjaxToolKit控件的Behavior！
Asp.net Ajax 编程备忘录----细数28个服务器端控件 [原创]一文中的确是忽略了这一点


 Accordion 行为标识 "_AccordionExtender";

Default constructor that tells ASP.NET to render it as a DIV

l             TransitionDuration  切换动画的长度               默认：500ms

l             FramesPerSecond   切换动画每秒多少帧           默认：15

l             FadeTransitions     切换过程中是否使用淡入淡出   默认：false

l             AutoSize           控件增长方式                默认：Autosize.None

l             SelectedIndex       展开的选项卡的Index               默认：0


页面脚本：
       <script language="javascript" type="text/javascript">
            function toggleFade() {
                var behavior = $find('ctl00_ContentPlaceHolder1_MyAccordion_AccordionExtender');
                if (behavior) {
                    behavior.set_FadeTransitions(!behavior.get_FadeTransitions());
                }
            }
            function changeAutoSize() {
                var behavior = $find('ctl00_ContentPlaceHolder1_MyAccordion_AccordionExtender');
                var ctrl = $get('autosize');
                if (behavior) {
                    var size = 'None';
                    switch (ctrl.selectedIndex) {
                        case 0 :
                            behavior.get_element().style.height = 'auto';
                            size = AjaxControlToolkit.AutoSize.None;
                            break;
                        case 1 :
                            behavior.get_element().style.height = '400px';
                            size = AjaxControlToolkit.AutoSize.Fill;
                            break;
                        case 2 :
                            behavior.get_element().style.height = '400px';
                            size = AjaxControlToolkit.AutoSize.Limit;
                            break;
                    }
                    behavior.set_AutoSize(size);
                }
                if (document.focus) {
                    document.focus();
                }
            }
        </script>