How to use async within a lambda which returns a collection

Question

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.

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

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.

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);

}

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

Cannot convert source type

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

Where am I going wrong here?

Solution

• 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);
}

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);
}

Fiddle: https://dotnetfiddle.net/78ApTI

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

Comments

[Grundy]

now your select return enumeration with task, but you try assign it to enumeration with ThingDetail, as vatiant you can use var keyword instead declare type directly like: var things=... instead of IEnumerable<ThingDetail> things = ...

[M.Babcock]

result isn't defined in the code block. Presumably, you meant it to be return Json(things, JsonRequestBehavior.AllowGet)?

[Program.X]

"That's the way to do it". Excellent, thanks. I had tried the WhenAll method but obviously had missed an interim step. I take your point about the Async method naming convention and I do agree. I'll implement this.

[M.Babcock]

Shouldn't the last line in both code blocks be return Json(things, JsonRequestBehavior.AllowGet);? result doesn't appear to be defined in either code block.

[dcastro]

@M.Babcock Correct, it's fixed now. Thanks