Async await in linq select

Question

I need to modify an existing program and it contains following code   var inputs   events Select async ev   gt  await ProcessEventAsync ev                       Select t   gt  t Result                      Where i   gt  i    null                      ToList      But this seems very weird to me  first of all the use of async and awaitin the select  According to this answer by Stephen Cleary I should be able to drop those    Then the second Select which selects the result  Doesn t this mean the task isn t async at all and is performed synchronously  so much effort for nothing   or will the task be performed asynchronously and when it s done the rest of the query is executed   Should I write the above code like following according to another answer by Stephen Cleary   var tasks   await Task WhenAll events Select ev   gt  ProcessEventAsync ev     var inputs   tasks Where result   gt  result    null  ToList      and is it completely the same like this   var inputs    await Task WhenAll events Select ev   gt  ProcessEventAsync ev                                             Where result   gt  result    null  ToList      While i m working on this project I d like to change the first code sample but I m not too keen on changing  apparantly working  async code  Maybe I m just worrying for nothing and all 3 code samples do exactly the same thing   ProcessEventsAsync looks like this   async Task lt InputResult gt  ProcessEventAsync InputEvent ev

User · Answer

Existing code is working  but is blocking the thread   Select async ev   gt  await ProcessEventAsync ev    creates a new Task for every event  but  Select t   gt  t Result   blocks the thread waiting for each new task to end  In the other hand your code produce the same result but keeps asynchronous  Just one comment on your first code  This line var tasks   await Task WhenAll events     will produce a single Task lt TResult   gt  so the variable should be named in singular  Finally your last code make the same but is more succinct  For reference  Task Wait   Task WhenAll

User · Answer

I have the same problem as  KTCheek in that I need it to execute sequentially  However I figured I would try using IAsyncEnumerable  introduced in  NET Core 3  and await foreach  introduced in C  8   Here s what I have come up with  public static class IEnumerableExtensions       public static async IAsyncEnumerable lt TResult gt  SelectAsync lt TSource  TResult gt  this IEnumerable lt TSource gt  source  Func lt TSource  Task lt TResult gt  gt  selector            foreach  var item in source                yield return await selector item                      public static class IAsyncEnumerableExtensions       public static async Task lt List lt TSource gt  gt  ToListAsync lt TSource gt  this IAsyncEnumerable lt TSource gt  source            var list   new List lt TSource gt              await foreach  var item in source                list Add item                      return list           This can be consumed by saying  var inputs   await events SelectAsync ev   gt  ProcessEventAsync ev   ToListAsync     Update  Alternatively you can add a reference to  quot System Linq Async quot  and then you can say  var inputs   await events      ToAsyncEnumerable        SelectAwait async ev   gt  await ProcessEventAsync ev        ToListAsync

User · Answer

I prefer this as an extension method   public static async Task lt IEnumerable lt T gt  gt  WhenAll lt T gt  this IEnumerable lt Task lt T gt  gt  tasks        return await Task WhenAll tasks       So that it is usable with method chaining   var inputs   await events    Select async ev   gt  await ProcessEventAsync ev      WhenAll

User · Answer

I used this code   public static async Task lt IEnumerable lt TResult gt  gt  SelectAsync lt TSource TResult gt  this IEnumerable lt TSource gt  source  Func lt TSource  Task lt TResult gt  gt  method          return await Task WhenAll source Select async s   gt  await method s         like this   var result   await sourceEnumerable SelectAsync async s  gt await someFunction s other params

User · Answer

var inputs   events Select async ev   gt  await ProcessEventAsync ev                       Select t   gt  t Result                      Where i   gt  i    null                      ToList         But this seems very weird to me  first of all the use of async and await in the select  According to this answer by Stephen Cleary I should be able to drop those    The call to Select is valid  These two lines are essentially identical   events Select async ev   gt  await ProcessEventAsync ev   events Select ev   gt  ProcessEventAsync ev      There s a minor difference regarding how a synchronous exception would be thrown from ProcessEventAsync  but in the context of this code it doesn t matter at all       Then the second Select which selects the result  Doesn t this mean the task isn t async at all and is performed synchronously  so much effort for nothing   or will the task be performed asynchronously and when it s done the rest of the query is executed    It means that the query is blocking  So it is not really asynchronous   Breaking it down   var inputs   events Select async ev   gt  await ProcessEventAsync ev     will first start an asynchronous operation for each event  Then this line                       Select t   gt  t Result    will wait for those operations to complete one at a time  first it waits for the first event s operation  then the next  then the next  etc    This is the part I don t care for  because it blocks and also would wrap any exceptions in AggregateException      and is it completely the same like this    var tasks   await Task WhenAll events Select ev   gt  ProcessEventAsync ev     var inputs   tasks Where result   gt  result    null  ToList     var inputs    await Task WhenAll events Select ev   gt  ProcessEventAsync ev                                             Where result   gt  result    null  ToList      Yes  those two examples are equivalent  They both start all asynchronous operations  events Select        then asynchronously wait for all the operations to complete in any order  await Task WhenAll        then proceed with the rest of the work  Where       Both of these examples are different from the original code  The original code is blocking and will wrap exceptions in AggregateException

User · Answer

I wanted to call Select      but ensure it ran in sequence because running in parallel would cause some other concurrency problems  so I ended up with this  I cannot call  Result because it will block the UI thread  public static class TaskExtensions       public static async Task lt IEnumerable lt TResult gt  gt  SelectInSequenceAsync lt TSource  TResult gt  this IEnumerable lt TSource gt  source  Func lt TSource  Task lt TResult gt  gt  asyncSelector                var result   new List lt TResult gt             foreach  var s in source                        result Add await asyncSelector s                               return result           Usage  var inputs   events SelectInSequenceAsync ev   gt  ProcessEventAsync ev                       Where i   gt  i    null                      ToList     I am aware that Task WhenAll is the way to go when we can run in parallel

User · Answer

quot Just because you can doesn t mean you should  quot  You can probably use async await in LINQ expressions such that it will behave exactly as you want it to  but will any other developer reading your code still understand its behavior and intent   In particular  Should the async operations be run in parallel or are they intentionally sequential  Did the original developer even think about it   This is also shown clearly by the question  which seems to have been asked by a developer trying to understand someone else s code  without knowing its intent  To make sure this does not happen again  it may be best to rewrite the LINQ expression as a loop statement  if possible

User · Answer

With current methods available in Linq it looks quite ugly   var tasks   items Select      async item   gt  new               Item   item          IsValid   await IsValid item          var tuples   await Task WhenAll tasks   var validItems   tuples      Where p   gt  p IsValid       Select p   gt  p Item       ToList      Hopefully following versions of  NET will come up with more elegant tooling to handle collections of tasks and tasks of collections

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