DropDownList ddl = new DropDownList();
ddl.ID = "d1";
ddl.AutoPostBack = true;
ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged);
ddl.Items.Add(new ListItem("a","1"));
ddl.Items.Add(new ListItem("b","2"));
 Panel1.Controls.Add(ddl)

 protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList ddl = (DropDownList)sender;
Response.Write(ddl.SelectedValue);
        }