Web Part Sharepoint - Dropdown SelectIndexChanged to set visibility not working

Question

I'm developing a web part for sharepoint 2010. I have a dropdown with an event. On change it should show one label or another, depending on the value but this is not working. I did put autopostback to true but i dont want the whole page to be loaded everytime i click on the dropdown.

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;
            }
        }
    }

Answer 1

You just need to wrap your dropdown in an update panel

appears to be a good example

text is not allowed between the opening and closing tags update panel