WaitAll vs WhenAll

Question

What is the difference between Task WaitAll   and Task WhenAll   from the Async CTP   Can you provide some sample code to illustrate the different use cases

User · Answer

What do they do    Internally both do the same thing      What s the difference    WaitAll is a blocking call WhenAll - not - code will continue executing     Use which when    WaitAll when cannot continue without having the result WhenAll when what just to be notified  not blocked

User · Answer

As an example of the difference -- if you have a task the does something with the UI thread  e g  a task that represents an animation in a Storyboard  if you Task WaitAll   then the UI thread is blocked and the UI is never updated   if you use await Task WhenAll   then the UI thread is not blocked  and the UI will be updated

User · Answer

While JonSkeet s answer explains the difference in a typically excellent way there is another difference  exception handling   Task WaitAll throws an AggregateException when any of the tasks throws and you can examine all thrown exceptions  The await in await Task WhenAll unwraps the AggregateException and  returns  only the first exception   When the program below executes with await Task WhenAll taskArray  the output is as follows   19 11 2016 12 18 37 AM  Task 1 started 19 11 2016 12 18 37 AM  Task 3 started 19 11 2016 12 18 37 AM  Task 2 started Caught Exception in Main at 19 11 2016 12 18 40 AM  Task 1 throwing at 19 11 2016 12 18 38 AM Done    When the program below is executed with Task WaitAll taskArray  the output is as follows   19 11 2016 12 19 29 AM  Task 1 started 19 11 2016 12 19 29 AM  Task 2 started 19 11 2016 12 19 29 AM  Task 3 started Caught AggregateException in Main at 19 11 2016 12 19 32 AM  Task 1 throwing at 19 11 2016 12 19 30 AM Caught AggregateException in Main at 19 11 2016 12 19 32 AM  Task 2 throwing at 19 11 2016 12 19 31 AM Caught AggregateException in Main at 19 11 2016 12 19 32 AM  Task 3 throwing at 19 11 2016 12 19 32 AM Done    The program   class MyAmazingProgram       public class CustomException   Exception               public CustomException String message    base message                         static void WaitAndThrow int id  int waitInMs                Console WriteLine    DateTime UtcNow   Task  id  started             Thread Sleep waitInMs           throw new CustomException   Task  id  throwing at  DateTime UtcNow                static void Main string   args                Task Run async      gt                        await MyAmazingMethodAsync               Wait                static async Task MyAmazingMethodAsync                 try                       Task   taskArray     Task Factory StartNew      gt  WaitAndThrow 1  1000                                     Task Factory StartNew      gt  WaitAndThrow 2  2000                                     Task Factory StartNew      gt  WaitAndThrow 3  3000                   Task WaitAll taskArray                 await Task WhenAll taskArray               Console WriteLine  This isn t going to happen                      catch  AggregateException ex                        foreach  var inner in ex InnerExceptions                                Console WriteLine   Caught AggregateException in Main at  DateTime UtcNow       inner Message                                   catch  Exception ex                        Console WriteLine   Caught Exception in Main at  DateTime UtcNow       ex Message                     Console WriteLine  Done             Console ReadLine

User · Answer

Task WaitAll blocks the current thread until everything has completed   Task WhenAll returns a task which represents the action of waiting until everything has completed   That means that from an async method  you can use   await Task WhenAll tasks         which means your method will continue when everything s completed  but you won t tie up a thread to just hang around until that time

[c#] WaitAll vs WhenAll

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