Synchronously waiting for an async operation and why does Wait freeze the program here

Question

Preface  I m looking for an explanation  not just a solution  I already know the solution   Despite having spent several days studying MSDN articles about the Task-based Asynchronous Pattern  TAP   async and await  I m still a bit confused about some of the finer details   I m writing a logger for Windows Store Apps  and I want to support both asynchronous and synchronous logging  The asynchronous methods follow the TAP  the synchronous ones should hide all this  and look and work like ordinary methods   This is the core method of asynchronous logging   private async Task WriteToLogAsync string text        StorageFolder folder   ApplicationData Current LocalFolder      StorageFile file   await folder CreateFileAsync  log log           CreationCollisionOption OpenIfExists       await FileIO AppendTextAsync file  text          Windows Storage Streams UnicodeEncoding Utf8       Now the corresponding synchronous method     Version 1   private void WriteToLog string text        Task task   WriteToLogAsync text       task Wait        This looks correct  but it does not work  The whole program freezes forever   Version 2   Hmm   Maybe the task was not started   private void WriteToLog string text        Task task   WriteToLogAsync text       task Start        task Wait        This throws InvalidOperationException  Start may not be called on a promise-style task   Version 3   Hmm   Task RunSynchronously sounds promising   private void WriteToLog string text        Task task   WriteToLogAsync text       task RunSynchronously        This throws InvalidOperationException  RunSynchronously may not be called on a task not bound to a delegate  such as the task returned from an asynchronous method   Version 4  the solution    private void WriteToLog string text        var task   Task Run async      gt    await WriteToLogAsync text           task Wait        This works  So  2 and 3 are the wrong tools  But 1  What s wrong with 1 and what s the difference to 4  What makes 1 cause a freeze  Is there some problem with the task object  Is there a non-obvious deadlock

User · Answer

With small custom synchronization context  sync function can wait for completion of async function  without creating deadlock  Here is small example for WinForms app    Imports System Threading Imports System Runtime CompilerServices  Public Class Form1      Private Sub Form1 Load sender As Object  e As EventArgs  Handles MyBase Load         SyncMethod       End Sub        waiting inside Sync method for finishing async method     Public Sub SyncMethod           Dim sc As New SC         sc WaitForTask AsyncMethod            sc Release       End Sub      Public Async Function AsyncMethod   As Task Of Boolean          Await Task Delay 1000          Return True     End Function  End Class  Public Class SC     Inherits SynchronizationContext      Dim OldContext As SynchronizationContext     Dim ContextThread As Thread      Sub New           OldContext   SynchronizationContext Current         ContextThread   Thread CurrentThread         SynchronizationContext SetSynchronizationContext Me      End Sub      Dim DataAcquired As New Object     Dim WorkWaitingCount As Long   0     Dim ExtProc As SendOrPostCallback     Dim ExtProcArg As Object       lt MethodImpl MethodImplOptions Synchronized  gt      Public Overrides Sub Post d As SendOrPostCallback  state As Object          Interlocked Increment WorkWaitingCount          Monitor Enter DataAcquired          ExtProc   d         ExtProcArg   state         AwakeThread           Monitor Wait DataAcquired          Monitor Exit DataAcquired      End Sub      Dim ThreadSleep As Long   0      Private Sub AwakeThread           If Interlocked Read ThreadSleep   gt  0 Then ContextThread Resume       End Sub      Public Sub WaitForTask Tsk As Task          Dim aw   Tsk GetAwaiter          If aw IsCompleted Then Exit Sub          While Interlocked Read WorkWaitingCount   gt  0 Or aw IsCompleted   False             If Interlocked Read WorkWaitingCount    0 Then                 Interlocked Increment ThreadSleep                  ContextThread Suspend                   Interlocked Decrement ThreadSleep              Else                 Interlocked Decrement WorkWaitingCount                  Monitor Enter DataAcquired                  Dim Proc   ExtProc                 Dim ProcArg   ExtProcArg                 Monitor Pulse DataAcquired                  Monitor Exit DataAcquired                  Proc ProcArg              End If         End While      End Sub       Public Sub Release            SynchronizationContext SetSynchronizationContext OldContext       End Sub  End Class

User · Answer

The await inside your asynchronous method is trying to come back to the UI thread   Since the UI thread is busy waiting for the entire task to complete  you have a deadlock   Moving the async call to Task Run   solves the issue  Because the async call is now running on a thread pool thread  it doesn t try to come back to the UI thread  and everything therefore works   Alternatively  you could call StartAsTask   ConfigureAwait false  before awaiting the inner operation to make it come back to the thread pool rather than the UI thread  avoiding the deadlock entirely

User · Answer

Calling async code from synchronous code can be quite tricky   I explain the full reasons for this deadlock on my blog  In short  there s a  context  that is saved by default at the beginning of each await and used to resume the method   So if this is called in an UI context  when the await completes  the async method tries to re-enter that context to continue executing  Unfortunately  code using Wait  or Result  will block a thread in that context  so the async method cannot complete   The guidelines to avoid this are    Use ConfigureAwait continueOnCapturedContext  false  as much as possible  This enables your async methods to continue executing without having to re-enter the context  Use async all the way  Use await instead of Result or Wait    If your method is naturally asynchronous  then you  probably  shouldn t expose a synchronous wrapper

User · Answer

Here is what I did  private void myEvent Handler object sender  SomeEvent e         I dont know how many times this event will fire   Task t   new Task      gt          if  something    true                 DoSomething e                   t RunSynchronously        working great and not blocking UI thread

[c#] Synchronously waiting for an async operation, and why does Wait() freeze the program here

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