编辑
我已经更改了问题的标题以反映我遇到的问题，同时也给出了如何轻松实现这一目标的答案。
我试图让第二种方法返回Task<TResult>而不是Task第一种方法，但是由于尝试修复它，我得到了一系列错误。
- 我
return之前加的await body(partition.Current); - 反过来，它要求我在下面添加一个退货声明，所以我在
return null下面添加 - 但是现在 select 语句抱怨它无法从查询中推断类型参数
- 我
Task.Run改为Task.Run<TResult>但没有成功。
我该如何解决？
第一种方法来自http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx，第二种方法是我试图创建的重载。
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);
}));
}
}
用法示例：
使用这种方法，我想并行和异步下载多个文件：
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;
}
}