c# - async within a LINQ code - Clarification? -

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almost every so's answer regarding topic , states :

linq doesn't work async

also :

i recommend not think of "using async within linq"

but in stephen's book there sample :

problem: have collection of tasks await, , want processing on each task after completes. however, want processing each 1 as as completes, not waiting of other tasks.

one of recommended solutions : 

static async task<int> delayandreturnasync(int val) {  await task.delay(timespan.fromseconds(val));  return val; }  // method prints "1", "2", , "3". static async task processtasksasync() {  // create sequence of tasks.  task<int> taska = delayandreturnasync(2);  task<int> taskb = delayandreturnasync(3);  task<int> taskc = delayandreturnasync(1);  var tasks = new[] { taska, taskb, taskc };  var processingtasks = tasks.select(async t =>     {     var result = await t;     trace.writeline(result);     }).toarray();  // await processing complete await task.whenall(processingtasks);  }

question #1:

i don't understand why async inside linq statement - work . didn't "don't think using async within linq" ?

question #2:

when control reaches await t here — happen? control leaves processtasksasync method ? or leaves anonymous method , continue iteration ?

i don't understand why async inside linq statement - work . didn't "don't think using async within linq" ?

async mostly doesn't work linq because ienumerable<t> extensions don't infer delegate type , defer action<t>. have no special understanding of task class. means actual async delegate becomes async void, bad. in case of enumerable.select, have overload returns func<t> (which in turn func<task> in our case), equivalent async task, hence works fine async use-cases.

when control reaches await t here — happen? control leaves processtasksasync method ?

no, doesn't. enumerable.select projecting elements in sequence. means each element in collection, await t yield control iterator, continue iterating elements. that's why later have await task.whenall, ensure elements have finished execution.


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