protected void Page_Load(object sender, EventArgs e)
        {
            DropDownList myDdl = new DropDownList();
            myDdl.Items.Add("Test");
            myDdl.Items.Add("Test2");
            myDdl.AutoPostBack = true;
            myDdl.SelectedIndexChanged += new EventHandler(SelectedIndexChanged);
            if (!this.Page.Form.Controls.Contains(myDdl))
            {
                this.Page.Form.Controls.Add(myDdl);
            }
        }

        void SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList ddl = sender as DropDownList;

            DropDownList myDdl1 = new DropDownList();
            myDdl1.Items.Add("Test3");
            myDdl1.Items.Add("Test4");
            myDdl1.AutoPostBack = true;
            myDdl1.SelectedIndexChanged += new EventHandler(SelectedIndexChanged);
            this.Page.Form.Controls.Add(myDdl1);
        }