使用WebClient.DownloadFileTaskAsync时，为什么有些文件导致0KB

Question

I'm trying to download multiple files from an FTP server using WebClient.DownloadFileTaskAsync and repeatedly have the issue that several files end up being 0KB.

I've tried different suggested solutions but I just don't manage to get all files. What am I doing wrong?

class Program
{
    static void Main()
    {
        Setup(); // This only sets up working folders etc

        Task t = ProcessAsync();
        t.ContinueWith(bl =>
        {
            if (bl.Status == TaskStatus.RanToCompletion)
                Logger.Info("All done.");
            else
                Logger.Warn("Something went wrong.");
        });
        t.Wait();
    }

    private static void Setup() {...}

    static async Task<bool> ProcessAsync()
    {
        var c = new Catalog();

        var maxItems = Settings.GetInt("maxItems");
        Logger.Info((maxItems == 0 ? "Processing all items" : "Processing first {0} items"), maxItems);

        await c.ProcessCatalogAsync(maxItems);
        return true; // This is not really used atm
    }
}

public class Catalog
{
    public async Task ProcessCatalogAsync(int maxItems)
    {
        var client = new Client();
        var d = await client.GetFoldersAsync(_remoteFolder, maxItems);
        var list = d as IList<string> ?? d.ToList();
        Logger.Info("Found {0} folders", list.Count());

        await ProcessFoldersAsync(list);
    }

    private async Task ProcessFoldersAsync(IEnumerable<string> list)
    {
        var client = new Client();
        foreach (var mFolder in list.Select(folder => _folder + "/" + folder))
        {
            var items = await client.GetItemsAsync(mFolder);
            var file = items.FirstOrDefault(n => n.ToLower().EndsWith(".xml"));

            if (string.IsNullOrEmpty(file))
            {
                Logger.Warn("No metadata file found in {0}", mFolder);
                continue;
            }

            await client.DownloadFileAsync(mFolder, file);

            // Continue processing the received file...
        }
    }
}

public class Client
{
    public async Task<IEnumerable<string>> GetItemsAsync(string subfolder)
    {
        return await GetFolderItemsAsync(subfolder, false);
    }

    public async Task<IEnumerable<string>> GetFoldersAsync(string subfolder, int maxItems)
    {
        var folders = await GetFolderItemsAsync(subfolder, true);
        return maxItems == 0 ? folders : folders.Take(maxItems);
    }

    private async Task<IEnumerable<string>> GetFolderItemsAsync(string subfolder, bool onlyFolders)
    {
        // Downloads folder contents using WebRequest
    }

    public async Task DownloadFileAsync(string path, string file)
    {
        var remote = new Uri("ftp://" + _hostname + path + "/" + file);
        var local = _workingFolder + @"\" + file;
        using (var ftpClient = new WebClient { Credentials = new NetworkCredential(_username, _password) })
        {
            ftpClient.DownloadFileCompleted += (sender, e) => DownloadFileCompleted(sender, e, file);
            await ftpClient.DownloadFileTaskAsync(remote, local);
        }
    }

    public void DownloadFileCompleted(object sender, AsyncCompletedEventArgs e, Uri remote, string local)
    {
        if (e.Error != null)
        {
            Logger.Warn("Failed downloading\n\t{0}\n\t{1}", file, e.Error.Message);
            return;
        }
        Logger.Info("Downloaded \n\t{1}", file);
    }
}

Answer 1

Seems like some of your tasks aren't completed. Try do like this (I do the same when putting bunch of files to ftp)

1. Create array of tasks for every file being downloaded. Run them in cycle like this:

    Task[] tArray = new Task[DictToUpload.Count]; foreach (var pair in DictToUpload) { tArray[i] = Task.Factory.StartNew(()=>{/* some stuff */}); } await Task.WhenAll(tArray); 

2. Use await Task.WhenAll(taskArray) instead of await each task. This guarantees all your tasks are completed.

3. Learn some peace of TPL ;)