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?

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 = ... — Grundy
– Grundy, Commented Apr 23, 2015 at 10:55
result isn't defined in the code block. Presumably, you meant it to be return Json(things, JsonRequestBehavior.AllowGet)? — M.Babcock
– M.Babcock, Commented Jan 24, 2018 at 18:40

dcastro · Accepted Answer · 2018-01-24 18:49:01Z

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.

"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.
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.
This method is an anti pattern. The reason in that you don't know how much items in a collection. It can overload ThreadPool. In this case you should use TPL or split items on small blocks and WaitAll in a loop limited number of tasks.

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