先在前臺寫下如下方法：

<script>

function setSessionValue(value) {
            __doPostBack('SetSessionPostBack', value);
        }

</script>

然後呼叫這個方法

如在寫一個MESSAGEBOX控制元件

RenderContents中寫入

 writer.Write("function submit(value, message, form) {" +
                 "var json = /'{/"button/":{/"value/":/"/' + value + /'/",/"id/":/"" + _controlNeedPrompt + "/"}}/';" +
                             "setSessionBehindValue(json);" +

           "}");

後臺

public static string ConfirmValue(Page page)
        {
            if(page.Session["MessageBoxControl"]!=null)
            {
                MessageBoxBehind messageBoxBehind = (MessageBoxBehind)page.Session["MessageBoxControl"];
                page.ClientScript.GetPostBackEventReference(messageBoxBehind, string.Empty);

                if (page.IsPostBack)
                {
                    string eventTarget = (page.Request["__EVENTTARGET"] == null) ? string.Empty : page.Request["__EVENTTARGET"];
                    string eventArgument = (page.Request["__EVENTARGUMENT"] == null) ? string.Empty : page.Request["__EVENTARGUMENT"];

                    if (eventTarget == "SetSessionBehindPostBack")
                        page.Session["SessionMessageBoxBehind"] = eventArgument;
                }
                else
                {
                    page.Session["SessionMessageBoxBehind"] = null;
                }

                if (page.Session["SessionMessageBoxBehind"] != null)
                {
                    return page.Session["SessionMessageBoxBehind"].ToString();
                }
                else
                {
                    return "Null";
                }
            }
            else
            {
                return "Null";
            }
        }