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背景：

每年我们都会运行并归档来自特定 ASP.Net 应用程序的所有报告。该档案提供了系统数据在一年中特定时间的“快照”。我们使用 .Net Forms 构建了一个 GUI，它使用 System.Net.WebClient 调用报表服务器并将报表下载到特定目录。这在过去运作良好。

今年我们将 Excel 文件包含在我们的档案中。Excel 文件由 .aspx 页面（Windows Sever 2003、IIS6、.Net 4.0）创建，该页面接受查询字符串，然后返回 Excel 文件。这适用于 100 个左右的 Excel 文件，但之后我们开始遇到问题。我们每年归档大约 300,000 个文件。

力学：

我们使用 WebClient.DownloadFileAsync 来拉取我们的文件，以免锁定 UI 线程。我们依靠 WebClient.DownloadFileCompleted 事件来告诉我们每个文件何时完成下载。当提出 DownloadFileCompleted 时，我们开始下载下一个文件。

问题：

我们正在锁定网络服务器。每个文件只需要几分之一秒的时间来下载，大约 167 个文件后，网络服务器锁定（页面超时），存档过程暂停几分钟。然后存档过程又下载了大约 100 个文件，并再次停止了几分钟。这会持续几个小时，直到归档过程开始每分钟左右抓取一个文件。

看来 IIS6 的线程已用完，但我们如何才能阻止这种情况发生呢？

以下是我们正在运行的代码的精简版本。我已经删除了与我们的问题无关的日志记录和其他项目。有没有人有任何提示？

public class DownloadExample
{
    private WebClient _WebClient = new WebClient();

    public string DownloadDirectory { get; set; }
    public List<Report> ReportList { get; set; }

    /// <summary>
    /// Constructor - sets all the attributes needed to access the report server, download, and archive the reports
    /// </summary>
    /// <param name="userName">Username</param>
    /// <param name="userPassword">Password for the user's domain username</param>
    /// <param name="userDomain">Domain of the username</param>
    /// <param name="downloadDirectory">Network path where the files will be archived</param>
    public DownloadExample(string userName, string userPassword, string userDomain, string downloadDirectory, List<Report> reportList)
    {
        DownloadDirectory = downloadDirectory;
        _WebClient.Credentials = new NetworkCredential(userName, userPassword, userDomain);
        _WebClient.DownloadFileCompleted += new System.ComponentModel.AsyncCompletedEventHandler(WebClient_DownloadFileCompleted);
        ReportList = reportList;
    }

    /// <summary>
    /// Kicks off the archive process
    /// </summary>
    public void StartDownloading()
    {
        if (ReportList.Count > 0)
        {
            Report rpt = ReportList[0];
            DoDownload(rpt.URL, CreateFileName(rpt), rpt.ReportTitle, rpt.ReportFormatType);
        }
    }

    /// <summary>
    /// Run the report and then download it to the archive directory
    /// </summary>
    /// <param name="url">URL of the Report</param>
    /// <param name="fileName">File name used to name the report file once it is downloaded</param>
    /// <param name="folderName">Name of the folder where the report will be downloaded to</param> 
    /// <param name="reportFormatType">Type of report being run, PDF or Excel</param>
    private bool DoDownload(string url, string fileName, string folderName, ReportFormatTypes reportFormatType)
    {
        bool isSuccess = false;

        string folderPath = DownloadDirectory + "\\" + folderName;
        DirectoryInfo dir = new DirectoryInfo(folderPath);
        if (!dir.Exists)
        {
            dir.Create();
            dir = null;
            dir = new DirectoryInfo(folderPath);
        }

        if (dir.Exists)
        {
            string path = folderPath + "\\" + fileName + ".xls";
            System.Uri uri = new Uri(url);
            try
            {
                _WebClient.DownloadFileAsync(uri, path);
            }
            catch (Exception exp)
            {
                //log error
            }
            FileInfo file = new FileInfo(path);
            isSuccess = file.Exists;
        }
        return isSuccess;
    }

    /// <summary>
    /// This event is fired after a file is downloaded
    /// After each file is downloaded, we remove the downloaded file from the list, 
    /// then download the next file.
    /// </summary>
    void WebClient_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
    {
        //Remove the report that was just run
        ReportList.RemoveAt(0);
        if (ReportList.Count > 0)
        {
            //Download the next report
            Report rpt = ReportList[0];
            DoDownload(rpt.URL, CreateFileName(rpt), rpt.ReportTitle, rpt.ReportFormatType);
        }
    }

    /// <summary>
    /// Does a bunch of stuff to create the file name...
    /// </summary>
    string CreateFileName(Report rpt)
    {
        return rpt.FileName;
    }
}
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