Web部件Sharepoint-下拉菜单SelectIndexChanged设置可见性不起作用

我正在为sharepoint 2010开发一个web部件。我有一个包含事件的下拉列表。在更改时，它应该显示一个或另一个标签，具体取决于值，但这不起作用。我确实将autopostback设置为true，但我不希望每次点击下拉菜单时都加载整个页面。

protected override void CreateChildControls()
    {
        toolPartPanel = new Panel();
        ddl = new DropDownList();
        ddl.ID = "ddl";
        ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged);
        ddl.EnableViewState = true;
        ddl.Items.Add("a");
        ddl.Items.Add("b");
        label1 = new Label();
        label1.Text = "a selected";
        label1.Visible = false;
        label2 = new Label();
        label2.Text = "b selected";
        label2.Visible = false;
        this.Controls.Add(label1); this.Controls.Add(label2);
        this.Controls.Add(ddl);
        this.Controls.Add(toolPartPanel);
        base.CreateChildControls();
    }
    void ddl_SelectedIndexChanged(object sender, EventArgs e) 
    {
        Selected = ddl.SelectedValue.ToString();
        if (Selected == "a")
        {
            label1.Visible = true;
        }
        else
        {
            if (Selected == "b")
            {
                label2.Visible = true;
            }
        }
    }