public void nextDown()
    {
        if (isStackDownload)
        {
            return;
        }
        if (downloadStack.Count <= 0)
        {
            return;
        }

        PhotoDownLoader pd = downloadStack[0];

        downloadStack.Remove(pd);
        if (pd == null || pd.url == "" || getName(pd.url) == "404")
        {
            nextDown();
            return;
        }
        else
        {
            pd.doDownLoad();
        }
    }