在CS里是这么写的。。可是却不能更新
 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
            BindDropDownList();

    }

    private void BindDropDownList()
    {
        DirectoryInfo di = new DirectoryInfo(Server.MapPath("image/face/"));
        FileInfo[] fi = di.GetFiles("*.jpg");
        foreach (FileInfo f in fi)
        {
            dropface.Items.Add(new ListItem(f.Name, "image/face/" + f.Name));
            ttxt.Text += "image/face/" + f.Name + '\n';
        }
        dropface.Attributes.Add("onchange", "documnet.getElementByld('imgface').src=documnet.getElementByld('" + dropface.ClientID + "').value; ");
      
    }

问题已解决！！