是否可以“等待收益返回 DoSomethingAsync()"? [英] Is it possible to "await yield return DoSomethingAsync()"

问题描述

常规迭代器块(即yield return")是否与async"和await"不兼容?

Are regular iterator blocks (i.e. "yield return") incompatible with "async" and "await"?

这很好地说明了我要做什么:

This gives a good idea of what I'm trying to do:

async Task<IEnumerable<Foo>> Method(String [] Strs)
{
    // I want to compose the single result to the final result, so I use the SelectMany
    var finalResult = UrlStrings.SelectMany(link =>   //i have an Urlstring Collection 
                   await UrlString.DownLoadHtmlAsync()  //download single result; DownLoadHtmlAsync method will Download the url's html code 
              );
     return finalResult;
}

但是，我收到一个编译器错误，指出无法从资源加载消息字符串".

However, I get a compiler error citing "unable to load message string from resources".

这是另一个尝试:

async Task<IEnumerable<Foo>> Method(String [] Strs)
{
    foreach(var str in strs)
    {
        yield return await DoSomethingAsync( str)
    }
}

但是，编译器再次返回错误:无法从资源加载消息字符串".

But again, the compiler returns an error: "unable to load message string from resources".

这是我项目中真正的编程代码

Here is the real programming code in my project

当我有一个列表任务时，这非常有用，该任务可以从 URL 下载 HTML我使用语法yield return await task"，结果是我想要IEnumerable.我不想写这段代码:

This is very useful when I have an List Task,that task can be download HTML from a URL and I use the syntax "yield return await task", the result is I want IEnumerable<Foo>. I don't want write this code:

async Task<IEnumerable<String>> DownLoadAllURL(String [] Strs)
{
    List<Foo> htmls= new ...
    foreach(var str in strs)
    {
        var html= await DownLoadHtmlAsync( str)
        htmls.Add(item)
    }
    return htmls;
}

但似乎我必须这样做.

感谢您的帮助.

推荐答案

您所描述的可以使用 Task.WhenAll 方法来完成.请注意代码如何变成简单的单行代码.发生的情况是每个单独的 url 开始下载，然后使用 WhenAll 将这些操作组合成一个可以等待的 Task.

What you are describing can be accomplished with the Task.WhenAll method. Notice how the code turns into a simple one-liner. What happens is that each individual url begins downloading and then WhenAll is used combine those operations into a single Task which can be awaited.

Task<IEnumerable<string>> DownLoadAllUrls(string[] urls)
{
    return Task.WhenAll(from url in urls select DownloadHtmlAsync(url));
}

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