如何在返回集合的lambda中使用异步 [英] How to use async within a lambda which returns a collection

问题描述

我有一个异步的上游"方法.我正在尝试遵循最佳做法，并在整个堆栈中一直保持全同步.

I have a method which is Async "upstream". I'm trying to follow best practice and go all-in qith async all the way up the stack.

如果我依靠.Result()，那么在MVC中的Controller动作中，我可以预料地遇到了死锁问题.

Within a Controller action within MVC I predictably hit the deadlock issue If I rely on .Result().

将Controller动作更改为异步似乎是一种方法，尽管问题是在lambda中多次调用了异步方法.

Changing the Controller action to async seems to be the way to go, though the issue is that the async method is called multiple times within a lambda.

我如何等待返回多个结果的lamda ?

How can I await on a lamda that returns multiple results?

public async Task<JsonResult>  GetLotsOfStuff()
{
    IEnumerable<ThingDetail> things=  previouslyInitialisedCollection
                                      .Select(async q => await GetDetailAboutTheThing(q.Id)));
    return Json(result, JsonRequestBehavior.AllowGet);

}

您可以看到我已经尝试使lambda异步，但这只是给编译器一个例外:

You can see I have tried making the lambda async, but this just gives a compiler exception:

无法转换源类型

System.Collections.Generic.IEnumerable<System.Threading.Tasks.Task<ThingDetail>定位到目标类型System.Collections.Generic.IEnumerable<ThingDetail>

System.Collections.Generic.IEnumerable<System.Threading.Tasks.Task<ThingDetail> to target type System.Collections.Generic.IEnumerable<ThingDetail>

我在哪里错了?

推荐答案

• 将您的Thing集合转换为Task<Thing>的集合.
• 然后使用Task.WhenAll加入所有这些任务，然后等待它.
• 等待联合任务将为您提供Thing[]
• Convert your collection of Things into a collection of Task<Thing>s.
• Then join all those tasks using Task.WhenAll and await it.
• Awaiting the joint task will give you a Thing[]




public async Task<JsonResult>  GetLotsOfStuff()
{
    IEnumerable<Task<ThingDetail>> tasks = collection.Select(q => GetDetailAboutTheThing(q.Id));

    Task<int[]> jointTask = Task.WhenAll(tasks);

    IEnumerable<ThingDetail> things = await jointTask;

    return Json(things, JsonRequestBehavior.AllowGet);
}

或者，简洁地使用类型推断:

Or, succinctly and using type inference:

public async Task<JsonResult>  GetLotsOfStuff()
{
    var tasks = collection.Select(q => GetDetailAboutTheThing(q.Id));
    var things = await Task.WhenAll(tasks);

    return Json(things, JsonRequestBehavior.AllowGet);
}

提琴: https://dotnetfiddle.net/78ApTI

注意:由于GetDetailAboutTheThing似乎返回了Task<Thing>，因此惯例是将Async附加到其名称-GetDetailAboutTheThingAsync.

Note: since GetDetailAboutTheThing seems to return a Task<Thing>, the convention is to append Async to its name - GetDetailAboutTheThingAsync.

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