Set it on every request to make it work on the first postback:

protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack == false)
        {
            i++;
            this.PreRender += new EventHandler(Cast_PreRender);
            this.ID = "Result" + i.ToString(); 
        }
PlayButton.ID = i.ToString();
        
    }