Task.StartNew Parallel.ForEach没有等待

Question

我有这个代码:

await Task.Factory.StartNew(
    () => Parallel.ForEach(
        urls,
        new ParallelOptions { MaxDegreeOfParallelism = 2 },
        async url =>
        {
           Uri uri = new Uri(url);
           string filename = System.IO.Path.GetFileName(uri.LocalPath);

           using (HttpClient client = new HttpClient())
           using (HttpResponseMessage response = await client.GetAsync(url))
           using (HttpContent content = response.Content)
           {
               // ... Read the string.
               using (var fileStream = new FileStream(config.M_F_P + filename, FileMode.Create, FileAccess.Write))
               {
                   await content.CopyToAsync(fileStream);
               }
           }
        }));

MessageBox.Show("Completed");

它应该处理超过800个元素的列表,但它不等待下载和文件写入完成.事实上,他开始下载和写作,显示消息,然后在后台继续下载...我需要下载很多文件并行和异步,但我必须等待所有这些文件下载.这段代码出了什么问题？

Answer 1

Parallel.ForEach不适用于异步.它期望一个Action但是为了等待它需要的异步方法Func<Task>.

您可以使用ActionBlock以异步构建的TPL Dataflow 代替.你给它一个委托(异或非)来对每个项目执行.您可以配置块的并行性(如果需要,还可以配置有界容量).并将您的商品发布到其中:

var block = new ActionBlock<string>(async url => 
{
    Uri uri = new Uri(url);
    string filename = System.IO.Path.GetFileName(uri.LocalPath);

    using (HttpClient client = new HttpClient())
    using (HttpResponseMessage response = await client.GetAsync(url))
    using (HttpContent content = response.Content)
    {
       // ... Read the string.
       using (var fileStream = new FileStream(config.M_F_P + filename, FileMode.Create, FileAccess.Write))
       {
           await content.CopyToAsync(fileStream);
       }
    }
}, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 2 } );

foreach (var url in urls)
{
    block.Post(url);
}

block.Complete();
await block.Completion;
// done