Running multiple async tasks and waiting for them all to complete

Question

I need to run multiple async tasks in a console application  and wait for them all to complete before further processing   There s many articles out there  but I seem to get more confused the more I read  I ve read and understand the basic principles of the Task library  but I m clearly missing a link somewhere   I understand that it s possible to chain tasks so that they start after another completes  which is pretty much the scenario for all the articles I ve read   but I want all my Tasks running at the same time  and I want to know once they re all completed   What s the simplest implementation for a scenario like this

User · Answer

This is how I do it with an array Func lt     var tasks   new Func lt Task gt              gt  myAsyncWork1            gt  myAsyncWork2            gt  myAsyncWork3       await Task WhenAll tasks Select task   gt  task    ToArray       Async     Task WaitAll tasks Select task   gt  task    ToArray       Or use WaitAll for Sync

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The best option I ve seen is the following extension method   public static Task ForEachAsync lt T gt  this IEnumerable lt T gt  sequence  Func lt T  Task gt  action        return Task WhenAll sequence Select action        Call it like this   await sequence ForEachAsync item   gt  item SomethingAsync blah      Or with an async lambda   await sequence ForEachAsync async item   gt        var more   await GetMoreAsync item       await more FrobbleAsync

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You could create many tasks like   List lt Task gt  TaskList   new List lt Task gt     foreach           var LastTask   new Task SomeFunction      LastTask Start       TaskList Add LastTask      Task WaitAll TaskList ToArray

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Do you want to chain the Tasks  or can they be invoked in a parallel manner   For chaining Just do something like  Task Run      ContinueWith      ContinueWith      ContinueWith       Task Factory StartNew      ContinueWith      ContinueWith      ContinueWith         and don t forget to check the previous Task instance in each ContinueWith as it might be faulted   For the parallel manner The most simple method I came across  Parallel Invoke Otherwise there s Task WaitAll or you can even use WaitHandles for doing a countdown to zero actions left  wait  there s a new class  CountdownEvent   or

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Both answers didn t mention the awaitable Task WhenAll  var task1   DoWorkAsync    var task2   DoMoreWorkAsync     await Task WhenAll task1  task2    The main difference between Task WaitAll and Task WhenAll is that the former will block  similar to using Wait on a single task  while the latter will not and can be awaited  yielding control back to the caller until all tasks finish  More so  exception handling differs  Task WaitAll   At least one of the Task instances was canceled -or- an exception was thrown during the execution of at least one of the Task instances  If a task was canceled  the AggregateException contains an OperationCanceledException in its InnerExceptions collection   Task WhenAll   If any of the supplied tasks completes in a faulted state  the returned task will also complete in a Faulted state  where its exceptions will contain the aggregation of the set of unwrapped exceptions from each of the supplied tasks  If none of the supplied tasks faulted but at least one of them was canceled  the returned task will end in the Canceled state  If none of the tasks faulted and none of the tasks were canceled  the resulting task will end in the RanToCompletion state  If the supplied array enumerable contains no tasks  the returned task will immediately transition to a RanToCompletion state before it s returned to the caller

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Yet another answer   but I usually find myself in a case  when I need to load data simultaneously and put it into variables  like   var cats   new List lt Cat gt     var dog   new Dog     var loadDataTasks   new Task         Task Run async      gt  cats   await LoadCatsAsync         Task Run async      gt  dog   await LoadDogAsync        try       await Task WhenAll loadDataTasks     catch  Exception ex           handle exception

User · Answer

There should be a more succinct solution than the accepted answer   It shouldn t take three steps to run multiple tasks simultaneously and get their results   Create tasks await Task WhenAll tasks  Get task results  e g   task1 Result   Here s a method that cuts this down to two steps      public async Task lt Tuple lt T1  T2 gt  gt  WhenAllGeneric lt T1  T2 gt  Task lt T1 gt  task1  Task lt T2 gt  task2                await Task WhenAll task1  task2           return Tuple Create task1 Result  task2 Result          You can use it like this  var taskResults   await Task WhenAll DoWorkAsync    DoMoreWorkAsync     var DoWorkResult   taskResults Result Item1  var DoMoreWorkResult   taskResults Result Item2   This removes the need for the temporary task variables   The problem with using this is that while it works for two tasks  you d need to update it for three tasks  or any other number of tasks   Also it doesn t work well if one of the tasks doesn t return anything   Really  the  Net library should provide something that can do this

User · Answer

You can use WhenAll which will return an awaitable Task or WaitAll which  has no return type and will block further code execution simular to Thread Sleep until all tasks are completed  canceled or faulted      Example  var tasks   new Task         TaskOperationOne        TaskOperationTwo       Task WaitAll tasks      or await Task WhenAll tasks     If you want to run the tasks in a praticular order you can get inspiration form this anwser

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I prepared a piece of code to show you how to use the task for some of these scenarios           method to run tasks in a parallel      public async Task RunMultipleTaskParallel Task   tasks             await Task WhenAll tasks                methode to run task one by one      public async Task RunMultipleTaskOneByOne Task   tasks                for  int i   0  i  lt  tasks Length - 1  i                await tasks i                method to run i task in parallel      public async Task RunMultipleTaskParallel Task   tasks  int i                var countTask   tasks Length          var remainTasks   0          do                       int toTake    countTask  lt  i    countTask   i              var limitedTasks   tasks Skip remainTasks                                       Take toTake               remainTasks    toTake              await RunMultipleTaskParallel limitedTasks ToArray               while  remainTasks  lt  countTask

[c#] Running multiple async tasks and waiting for them all to complete

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