为什么Task.WhenAll返回void? [英] Why does Task.WhenAll return void?

问题描述

这是上图中的代码:

if (claims != null && claims.Any())
{
    // firstly, why doesn't this work?
    // var _claimResults = from claim in claims select UserManager.AddClaimAsync(user.Id, claim);

    // but this does...
    List<Task> _claimResults = new List<Task>();
    foreach (var claim in claims)
    {
        _claimResults.Add(UserManager.AddClaimAsync(user.Id, claim));   
    }

    // secondly, why does Task.WhenAll return void when it clearly says it returns Task?
    Task claimsResult = await Task.WhenAll(_claimResults);
}

1. 为什么LINQ表达式不起作用，而foreach却起作用. LINQ表达式在执行时给我一个底层提供者无法打开"异常.
2. 为什么Task.WhenAll()声明返回类型为Task时返回void?

claims是一个List<Claim>，我认为是List<System.Security.Claim>.

claims is a List<Claim> which I think is List<System.Security.Claim>.

推荐答案

WhenAll返回Task，但是您正在等待该任务.等待普通的Task(而不是Task<T>)不会产生任何结果.所以你要么想要:

WhenAll returns a Task, but then you're awaiting that task. Awaiting a plain Task (rather than a Task<T>) gives no result. So you either want:

Task claimsResult = Task.WhenAll(_claimResults);

或

await Task.WhenAll(_claimResults);

我怀疑LINQ问题是因为您的foreach方法立即实现了查询-LINQ等效项是:

My suspicion is that the LINQ problem is because your foreach approach materializes the query immediately - the LINQ equivalent would be:

var _claimsResults = claims.Select(claim => UserManager.AddClaimAsync(user.Id, claim))
                           .ToList();

...，其中ToList()方法立即实现结果. (我使用方法调用语法而不是查询表达式语法，因为对于像这样的琐碎查询，查询表达式是毫无意义的.)

... where the ToList() method materializes the results immediately. (I've used the method call syntax rather than query expression syntax because query expressions are pretty pointless for trivial queries like this.)

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