How to avoid the 0-byte file on Webclient download error

_client.DownloadDataAsync(new Uri(url));
_client.DownloadDataCompleted += (sender, e) =>
{
    try
    {
        if (e.Result != null && e.Result.Length > 0 && e.Error == null && e.Cancelled == false)
            using (FileStream fileStream = new FileStream(path + file_name, FileMode.Create))
                fileStream.Write(e.Result, 0, e.Result.Length);
    }
    catch (Exception ex)
    {
        OpenOperator.Error(string.Format("列表页生成任务异常，url：{0}", url), ex);
    }
};

大规模download会涉及到站点并发问题，请参见：

.Net魔法堂：开启IIS的WebGarden、WebFarm和StateServer之旅