异步和并行下载文件

public static class Extensions
{
    public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
    {
        return Task.WhenAll(
            from partition in Partitioner.Create(source).GetPartitions(dop)
            select Task.Run(async delegate
            {
                using (partition)
                    while (partition.MoveNext())
                        await body(partition.Current);
            }));
    }
    public static Task ForEachAsync<T, TResult>(this IEnumerable<T> source, int dop, Func<T, Task<TResult>> body)
    {
        return Task.WhenAll(
            from partition in Partitioner.Create(source).GetPartitions(dop)
            select Task.Run(async delegate
            {
                using (partition)
                    while (partition.MoveNext())
                        await body(partition.Current);
            }));
    }
}

private async void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    Artist artist = await GetArtist();
    IEnumerable<string> enumerable = artist.Reviews.Select(s => s.ImageUrl);
    string[] downloadFile = await DownloadFiles(enumerable);
}
public static async Task<string[]> DownloadFiles(IEnumerable<string> enumerable)
{
    if (enumerable == null) throw new ArgumentNullException("enumerable");
    await enumerable.ForEachAsync(5, s => DownloadFile(s));
    // Incomplete, the above statement is void and can't be returned
}
public static async Task<string> DownloadFile(string address)
{
    /* Download a file from specified address, 
        * return destination file name on success or null on failure */
    if (address == null)
    {
        return null;
    }
    Uri result;
    if (!Uri.TryCreate(address, UriKind.Absolute, out result))
    {
        Debug.WriteLine(string.Format("Couldn't create URI from specified address: {0}", address));
        return null;
    }
    try
    {
        using (var client = new WebClient())
        {
            string fileName = Path.GetTempFileName();
            await client.DownloadFileTaskAsync(address, fileName);
            Debug.WriteLine(string.Format("Downloaded file saved to: {0} ({1})", fileName, address));
            return fileName;
        }
    }
    catch (WebException webException)
    {
        Debug.WriteLine(string.Format("Couldn't download file from specified address: {0}", webException.Message));
        return null;
    }
}

我解决了它并在此处发布，可能会帮助任何遇到相同问题的人。

我最初需要的是一个小助手，它可以快速下载图像，但如果服务器没有快速响应，也会断开连接，所有这些都是并行和异步的。

此帮助程序将返回一个元组，其中包含远程路径，本地路径和异常(如果发生异常(;非常有用，因为知道为什么错误的下载出错总是好的。我想我没有忘记下载可能发生的任何情况，但欢迎您发表评论。

• 指定要下载的 URL 列表
• 您可以指定保存该文件名的本地文件名，如果没有为您生成一个
(
• 可选(取消下载的持续时间(方便慢速或无法访问的服务器(

您可以只使用DownloadFileTaskAsync本身，也可以使用 ForEachAsync 帮助程序进行并行和异步下载。

代码带有如何使用它的示例：

private async void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    IEnumerable<string> enumerable = your urls here;
    var results = new List<Tuple<string, string, Exception>>();
    await enumerable.ForEachAsync(s => DownloadFileTaskAsync(s, null, 1000), (url, t) => results.Add(t));
}
/// <summary>
///     Downloads a file from a specified Internet address.
/// </summary>
/// <param name="remotePath">Internet address of the file to download.</param>
/// <param name="localPath">
///     Local file name where to store the content of the download, if null a temporary file name will
///     be generated.
/// </param>
/// <param name="timeOut">Duration in miliseconds before cancelling the  operation.</param>
/// <returns>A tuple containing the remote path, the local path and an exception if one occurred.</returns>
private static async Task<Tuple<string, string, Exception>> DownloadFileTaskAsync(string remotePath,
    string localPath = null, int timeOut = 3000)
{
    try
    {
        if (remotePath == null)
        {
            Debug.WriteLine("DownloadFileTaskAsync (null remote path): skipping");
            throw new ArgumentNullException("remotePath");
        }
        if (localPath == null)
        {
            Debug.WriteLine(
                string.Format(
                    "DownloadFileTaskAsync (null local path): generating a temporary file name for {0}",
                    remotePath));
            localPath = Path.GetTempFileName();
        }
        using (var client = new WebClient())
        {
            TimerCallback timerCallback = c =>
            {
                var webClient = (WebClient) c;
                if (!webClient.IsBusy) return;
                webClient.CancelAsync();
                Debug.WriteLine(string.Format("DownloadFileTaskAsync (time out due): {0}", remotePath));
            };
            using (var timer = new Timer(timerCallback, client, timeOut, Timeout.Infinite))
            {
                await client.DownloadFileTaskAsync(remotePath, localPath);
            }
            Debug.WriteLine(string.Format("DownloadFileTaskAsync (downloaded): {0}", remotePath));
            return new Tuple<string, string, Exception>(remotePath, localPath, null);
        }
    }
    catch (Exception ex)
    {
        return new Tuple<string, string, Exception>(remotePath, null, ex);
    }
}
public static class Extensions
{
    public static Task ForEachAsync<TSource, TResult>(
        this IEnumerable<TSource> source,
        Func<TSource, Task<TResult>> taskSelector, Action<TSource, TResult> resultProcessor)
    {
        var oneAtATime = new SemaphoreSlim(5, 10);
        return Task.WhenAll(
            from item in source
            select ProcessAsync(item, taskSelector, resultProcessor, oneAtATime));
    }
    private static async Task ProcessAsync<TSource, TResult>(
        TSource item,
        Func<TSource, Task<TResult>> taskSelector, Action<TSource, TResult> resultProcessor,
        SemaphoreSlim oneAtATime)
    {
        TResult result = await taskSelector(item);
        await oneAtATime.WaitAsync();
        try
        {
            resultProcessor(item, result);
        }
        finally
        {
            oneAtATime.Release();
        }
    }
}

我没有更改ForEachAsync的签名来选择并行级别，我会让您根据需要进行调整。

输出示例：

DownloadFileTaskAsync (null local path): generating a temporary file name for http://cache.thephoenix.com/secure/uploadedImages/The_Phoenix/Music/CD_Review/main_OTR_Britney480.jpg
DownloadFileTaskAsync (null local path): generating a temporary file name for http://ssimg.soundspike.com/artists/britneyspears_femmefatale_cd.jpg
DownloadFileTaskAsync (null local path): generating a temporary file name for http://a323.yahoofs.com/ymg/albumreviewsuk__1/albumreviewsuk-526650850-1301400550.jpg?ymm_1xEDE5bu0tMi
DownloadFileTaskAsync (null remote path): skipping
DownloadFileTaskAsync (time out due): http://hangout.altsounds.com/geek/gars/images/3/9/8/5/2375.jpg
DownloadFileTaskAsync (time out due): http://www.beat.com.au/sites/default/files/imagecache/630_315sr/images/article/header/2011/april/britney-spears-femme-fatale.jpg
DownloadFileTaskAsync (time out due): http://cache.thephoenix.com/secure/uploadedImages/The_Phoenix/Music/CD_Review/main_OTR_Britney480.jpg
DownloadFileTaskAsync (downloaded): http://newblog.thecmuwebsite.com/wp-content/uploads/2009/12/britneyspears1.jpg
DownloadFileTaskAsync (downloaded): http://newblog.thecmuwebsite.com/wp-content/uploads/2009/12/britneyspears1.jpg
DownloadFileTaskAsync (downloaded): http://static.guim.co.uk/sys-images/Music/Pix/site_furniture/2011/3/22/1300816812640/Femme-Fatale.jpg
DownloadFileTaskAsync (downloaded): http://www.sputnikmusic.com/images/albums/72328.jpg

过去需要 1 分钟的时间现在只需 10 秒即可获得相同的结果:)

非常感谢这 2 篇帖子的作者：

http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx

http://blogs.msdn.com/b/pfxteam/archive/2012/03/04/10277325.aspx

对于像我这样最近来到这里的人，我刚刚发现在 .NET 6：https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.parallel.foreachasync?view=net-6.0 中实现了Parallel.ForEachAsync。

它的用法就像你期望的那样，它的优点是允许指定并行度。例如：

var someInputData = new []
{
    "someData1",
    "someData2",
    "someData3"
};
ParallelOptions parallelOptions = new()
{
    MaxDegreeOfParallelism = 3
};
 
await Parallel.ForEachAsync(someInputData, parallelOptions, 
    async (input, cancellationToken) => 
    {
       // Some async Func. 
       // The Func can make use of:
       // - the `input` variable, which will contain the element in the `someInputData` list;
       // - the `cancellationToken` variable, usable to cancel the async operation.
    });

另请参阅 https://www.hanselman.com/blog/parallelforeachasync-in-net-6。