我不知道这是否会比您的解决方案更好，但是您是否尝试过？

if (ScriptManager.GetCurrent(Page).IsInAsyncPostBack)
{
    Control ctrl = GetControlThatCausedPostBack(Page);
    if (ctrl is UpdatePanel)
    {
        //handle updatepanel postback
    }
}

private Control GetControlThatCausedPostBack(Page page)
{
    //initialize a control and set it to null
    Control ctrl = null;

    //get the event target name and find the control
    string ctrlName = Page.Request.Params.Get("__EVENTTARGET");
    if (!String.IsNullOrEmpty(ctrlName))
        ctrl = page.FindControl(ctrlName);

    //return the control to the calling method
    return ctrl;
}