Using async await for multiple tasks

Question

I m using an API client that is completely asynchrounous  that is  each operation either returns Task or Task lt T gt   e g   static async Task DoSomething int siteId  int postId  IBlogClient client        await client DeletePost siteId  postId      call API client     Console WriteLine  Deleted post  0     siteId       Using the C  5 async await operators  what is the correct most efficient way to start multiple tasks and wait for them all to complete   int   ids   new     1  2  3  4  5    Parallel ForEach ids  i   gt  DoSomething 1  i  blogClient  Wait       or   int   ids   new     1  2  3  4  5    Task WaitAll ids Select i   gt  DoSomething 1  i  blogClient   ToArray       Since the API client is using HttpClient internally  I would expect this to issue 5 HTTP requests immediately  writing to the console as each one completes

User · Accepted Answer

int   ids   new     1  2  3  4  5    Parallel ForEach ids  i   gt  DoSomething 1  i  blogClient  Wait       Although you run the operations in parallel with the above code  this code blocks each thread that each operation runs on  For example  if the network call takes 2 seconds  each thread hangs for 2 seconds w o doing anything but waiting   int   ids   new     1  2  3  4  5    Task WaitAll ids Select i   gt  DoSomething 1  i  blogClient   ToArray       On the other hand  the above code with WaitAll also blocks the threads and your threads won t be free to process any other work till the operation ends    Recommended Approach  I would prefer WhenAll which will perform your operations asynchronously in Parallel   public async Task DoWork          int   ids   new     1  2  3  4  5        await Task WhenAll ids Select i   gt  DoSomething 1  i  blogClient            In fact  in the above case  you don t even need to await  you can just directly return from the method as you don t have any continuations   public Task DoWork          int   ids   new     1  2  3  4  5        return Task WhenAll ids Select i   gt  DoSomething 1  i  blogClient          To back this up  here is a detailed blog post going through all the alternatives and their advantages disadvantages  How and Where Concurrent Asynchronous I O with ASP NET Web API

User · Answer

Since the API you re calling is async  the Parallel ForEach version doesn t make much sense  You shouldnt use  Wait in the WaitAll version since that would lose the parallelism  Another alternative if the caller is async is using Task WhenAll after doing Select and ToArray to generate the array of tasks  A second alternative is using Rx 2 0

User · Answer

I was curious to see the results of the methods provided in the question as well as the accepted answer  so I put it to the test   Here s the code   using System  using System Collections Generic  using System Linq  using System Threading  using System Threading Tasks   namespace AsyncTest       class Program               class Worker                       public int Id              public int SleepTimeout               public async Task DoWork DateTime testStart                                var workerStart   DateTime Now                  Console WriteLine  Worker  0  started on thread  1   beginning  2  seconds after test start                        Id  Thread CurrentThread ManagedThreadId   workerStart-testStart  TotalSeconds ToString  F2                     await Task Run      gt  Thread Sleep SleepTimeout                    var workerEnd   DateTime Now                  Console WriteLine  Worker  0  stopped  the worker took  1  seconds  and it finished  2  seconds after the test start                       Id   workerEnd-workerStart  TotalSeconds ToString  F2     workerEnd-testStart  TotalSeconds ToString  F2                                      static void Main string   args                        var workers   new List lt Worker gt                                new Worker   Id   1  SleepTimeout   1000                    new Worker   Id   2  SleepTimeout   2000                    new Worker   Id   3  SleepTimeout   3000                    new Worker   Id   4  SleepTimeout   4000                    new Worker   Id   5  SleepTimeout   5000                                var startTime   DateTime Now              Console WriteLine  Starting test  Parallel ForEach                   PerformTest ParallelForEach workers  startTime               var endTime   DateTime Now              Console WriteLine  Test finished after  0  seconds  n                    endTime - startTime  TotalSeconds ToString  F2                  startTime   DateTime Now              Console WriteLine  Starting test  Task WaitAll                   PerformTest TaskWaitAll workers  startTime               endTime   DateTime Now              Console WriteLine  Test finished after  0  seconds  n                    endTime - startTime  TotalSeconds ToString  F2                  startTime   DateTime Now              Console WriteLine  Starting test  Task WhenAll                   var task   PerformTest TaskWhenAll workers  startTime               task Wait                endTime   DateTime Now              Console WriteLine  Test finished after  0  seconds  n                    endTime - startTime  TotalSeconds ToString  F2                  Console ReadKey                       static void PerformTest ParallelForEach List lt Worker gt  workers  DateTime testStart                        Parallel ForEach workers  worker   gt  worker DoWork testStart  Wait                        static void PerformTest TaskWaitAll List lt Worker gt  workers  DateTime testStart                        Task WaitAll workers Select worker   gt  worker DoWork testStart   ToArray                        static Task PerformTest TaskWhenAll List lt Worker gt  workers  DateTime testStart                        return Task WhenAll workers Select worker   gt  worker DoWork testStart                         And the resulting output   Starting test  Parallel ForEach    Worker 1 started on thread 1  beginning 0 21 seconds after test start  Worker 4 started on thread 5  beginning 0 21 seconds after test start  Worker 2 started on thread 3  beginning 0 21 seconds after test start  Worker 5 started on thread 6  beginning 0 21 seconds after test start  Worker 3 started on thread 4  beginning 0 21 seconds after test start  Worker 1 stopped  the worker took 1 90 seconds  and it finished 2 11 seconds after the test start  Worker 2 stopped  the worker took 3 89 seconds  and it finished 4 10 seconds after the test start  Worker 3 stopped  the worker took 5 89 seconds  and it finished 6 10 seconds after the test start  Worker 4 stopped  the worker took 5 90 seconds  and it finished 6 11 seconds after the test start  Worker 5 stopped  the worker took 8 89 seconds  and it finished 9 10 seconds after the test start  Test finished after 9 10 seconds   Starting test  Task WaitAll    Worker 1 started on thread 1  beginning 0 01 seconds after test start  Worker 2 started on thread 1  beginning 0 01 seconds after test start  Worker 3 started on thread 1  beginning 0 01 seconds after test start  Worker 4 started on thread 1  beginning 0 01 seconds after test start  Worker 5 started on thread 1  beginning 0 01 seconds after test start  Worker 1 stopped  the worker took 1 00 seconds  and it finished 1 01 seconds after the test start  Worker 2 stopped  the worker took 2 00 seconds  and it finished 2 01 seconds after the test start  Worker 3 stopped  the worker took 3 00 seconds  and it finished 3 01 seconds after the test start  Worker 4 stopped  the worker took 4 00 seconds  and it finished 4 01 seconds after the test start  Worker 5 stopped  the worker took 5 00 seconds  and it finished 5 01 seconds after the test start  Test finished after 5 01 seconds   Starting test  Task WhenAll    Worker 1 started on thread 1  beginning 0 00 seconds after test start  Worker 2 started on thread 1  beginning 0 00 seconds after test start  Worker 3 started on thread 1  beginning 0 00 seconds after test start  Worker 4 started on thread 1  beginning 0 00 seconds after test start  Worker 5 started on thread 1  beginning 0 00 seconds after test start  Worker 1 stopped  the worker took 1 00 seconds  and it finished 1 00 seconds after the test start  Worker 2 stopped  the worker took 2 00 seconds  and it finished 2 00 seconds after the test start  Worker 3 stopped  the worker took 3 00 seconds  and it finished 3 00 seconds after the test start  Worker 4 stopped  the worker took 4 00 seconds  and it finished 4 00 seconds after the test start  Worker 5 stopped  the worker took 5 00 seconds  and it finished 5 00 seconds after the test start  Test finished after 5 00 seconds

User · Answer

I just want to add to all great answers above  that if you write a library it s a good practice to use ConfigureAwait false  and get better performance  as said here  So this snippet seems to be better   public static async Task DoWork            int   ids   new     1  2  3  4  5         await Task WhenAll ids Select i   gt  DoSomething 1  i    ConfigureAwait false       A full fiddle link here

User · Answer

Parallel ForEach requires a list of user-defined workers and a non-async Action to perform with each worker   Task WaitAll and Task WhenAll require a List lt Task gt   which are by definition asynchronous   I found RiaanDP s response very useful to understand the difference  but it needs a correction for Parallel ForEach  Not enough reputation to respond to his comment  thus my own response   using System  using System Collections Generic  using System Linq  using System Threading  using System Threading Tasks   namespace AsyncTest       class Program               class Worker                       public int Id              public int SleepTimeout               public void DoWork DateTime testStart                                var workerStart   DateTime Now                  Console WriteLine  Worker  0  started on thread  1   beginning  2  seconds after test start                        Id  Thread CurrentThread ManagedThreadId   workerStart - testStart  TotalSeconds ToString  F2                     Thread Sleep SleepTimeout                   var workerEnd   DateTime Now                  Console WriteLine  Worker  0  stopped  the worker took  1  seconds  and it finished  2  seconds after the test start                       Id   workerEnd - workerStart  TotalSeconds ToString  F2     workerEnd - testStart  TotalSeconds ToString  F2                                public async Task DoWorkAsync DateTime testStart                                var workerStart   DateTime Now                  Console WriteLine  Worker  0  started on thread  1   beginning  2  seconds after test start                        Id  Thread CurrentThread ManagedThreadId   workerStart - testStart  TotalSeconds ToString  F2                     await Task Run      gt  Thread Sleep SleepTimeout                    var workerEnd   DateTime Now                  Console WriteLine  Worker  0  stopped  the worker took  1  seconds  and it finished  2  seconds after the test start                       Id   workerEnd - workerStart  TotalSeconds ToString  F2     workerEnd - testStart  TotalSeconds ToString  F2                                      static void Main string   args                        var workers   new List lt Worker gt                                new Worker   Id   1  SleepTimeout   1000                    new Worker   Id   2  SleepTimeout   2000                    new Worker   Id   3  SleepTimeout   3000                    new Worker   Id   4  SleepTimeout   4000                    new Worker   Id   5  SleepTimeout   5000                                var startTime   DateTime Now              Console WriteLine  Starting test  Parallel ForEach                   PerformTest ParallelForEach workers  startTime               var endTime   DateTime Now              Console WriteLine  Test finished after  0  seconds  n                    endTime - startTime  TotalSeconds ToString  F2                  startTime   DateTime Now              Console WriteLine  Starting test  Task WaitAll                   PerformTest TaskWaitAll workers  startTime               endTime   DateTime Now              Console WriteLine  Test finished after  0  seconds  n                    endTime - startTime  TotalSeconds ToString  F2                  startTime   DateTime Now              Console WriteLine  Starting test  Task WhenAll                   var task   PerformTest TaskWhenAll workers  startTime               task Wait                endTime   DateTime Now              Console WriteLine  Test finished after  0  seconds  n                    endTime - startTime  TotalSeconds ToString  F2                  Console ReadKey                       static void PerformTest ParallelForEach List lt Worker gt  workers  DateTime testStart                        Parallel ForEach workers  worker   gt  worker DoWork testStart                       static void PerformTest TaskWaitAll List lt Worker gt  workers  DateTime testStart                        Task WaitAll workers Select worker   gt  worker DoWorkAsync testStart   ToArray                        static Task PerformTest TaskWhenAll List lt Worker gt  workers  DateTime testStart                        return Task WhenAll workers Select worker   gt  worker DoWorkAsync testStart                         The resulting output is below  Execution times are comparable  I ran this test while my computer was doing the weekly anti virus scan  Changing the order of the tests did change the execution times on them   Starting test  Parallel ForEach    Worker 1 started on thread 9  beginning 0 02 seconds after test start  Worker 2 started on thread 10  beginning 0 02 seconds after test start  Worker 3 started on thread 11  beginning 0 02 seconds after test start  Worker 4 started on thread 13  beginning 0 03 seconds after test start  Worker 5 started on thread 14  beginning 0 03 seconds after test start  Worker 1 stopped  the worker took 1 00 seconds  and it finished 1 02 seconds after the test start  Worker 2 stopped  the worker took 2 00 seconds  and it finished 2 02 seconds after the test start  Worker 3 stopped  the worker took 3 00 seconds  and it finished 3 03 seconds after the test start  Worker 4 stopped  the worker took 4 00 seconds  and it finished 4 03 seconds after the test start  Worker 5 stopped  the worker took 5 00 seconds  and it finished 5 03 seconds after the test start  Test finished after 5 03 seconds   Starting test  Task WaitAll    Worker 1 started on thread 9  beginning 0 00 seconds after test start  Worker 2 started on thread 9  beginning 0 00 seconds after test start  Worker 3 started on thread 9  beginning 0 00 seconds after test start  Worker 4 started on thread 9  beginning 0 00 seconds after test start  Worker 5 started on thread 9  beginning 0 01 seconds after test start  Worker 1 stopped  the worker took 1 00 seconds  and it finished 1 01 seconds after the test start  Worker 2 stopped  the worker took 2 00 seconds  and it finished 2 01 seconds after the test start  Worker 3 stopped  the worker took 3 00 seconds  and it finished 3 01 seconds after the test start  Worker 4 stopped  the worker took 4 00 seconds  and it finished 4 01 seconds after the test start  Worker 5 stopped  the worker took 5 00 seconds  and it finished 5 01 seconds after the test start  Test finished after 5 01 seconds   Starting test  Task WhenAll    Worker 1 started on thread 9  beginning 0 00 seconds after test start  Worker 2 started on thread 9  beginning 0 00 seconds after test start  Worker 3 started on thread 9  beginning 0 00 seconds after test start  Worker 4 started on thread 9  beginning 0 00 seconds after test start  Worker 5 started on thread 9  beginning 0 00 seconds after test start  Worker 1 stopped  the worker took 1 00 seconds  and it finished 1 00 seconds after the test start  Worker 2 stopped  the worker took 2 00 seconds  and it finished 2 00 seconds after the test start  Worker 3 stopped  the worker took 3 00 seconds  and it finished 3 00 seconds after the test start  Worker 4 stopped  the worker took 4 00 seconds  and it finished 4 00 seconds after the test start  Worker 5 stopped  the worker took 5 00 seconds  and it finished 5 01 seconds after the test start  Test finished after 5 01 seconds

User · Answer

You can use Task WhenAll function that you can pass n tasks  Task WhenAll will return a task which runs to completion when all the tasks that you passed to Task WhenAll complete  You have to wait asynchronously on Task WhenAll so that you ll not block your UI thread      public async Task DoSomeThing             var Task   tasks   new Task numTasks          for int i   0  i  lt  numTask  i                       tasks i    CallSomeAsync                    await Task WhenAll tasks             code that ll execute on UI thread

[c#] Using async/await for multiple tasks

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