Catch an exception thrown by an async void method

Question

Using the async CTP from Microsoft for  NET  is it possible to catch an exception thrown by an async method in the calling method   public async void Foo         var x   await DoSomethingAsync            Handle the result  but sometimes an exception might be thrown         For example  DoSomethingAsync gets data from the network        and the data is invalid    a ProtocolException might be thrown        public void DoFoo         try               Foo              catch  ProtocolException ex                     The exception will never be caught               Instead when in debug mode  VS2010 will warn and continue               The deployed the app will simply crash               So basically I want the exception from the async code to bubble up into my calling code if that is even possible at all

User · Answer

The reason the exception is not caught is because the Foo() method has a void return type and so when await is called, it simply returns. As DoFoo() is not awaiting the completion of Foo, the exception handler cannot be used.

This opens up a simpler solution if you can change the method signatures - alter Foo() so that it returns type Task and then DoFoo() can await Foo(), as in this code:

public async Task Foo() {
    var x = await DoSomethingThatThrows();
}

public async void DoFoo() {
    try {
        await Foo();
    } catch (ProtocolException ex) {
        // This will catch exceptions from DoSomethingThatThrows
    }
}

User · Answer

Your code doesn t do what you might think it does  Async methods return immediately after the method begins waiting for the async result  It s insightful to use tracing in order to investigate how the code is actually behaving   The code below does the following    Create 4 tasks Each task will asynchronously increment a number and return the incremented number When the async result has arrived it is traced     nbsp   static TypeHashes  type   new TypeHashes typeof Program            private void Run         TracerConfig Reset  debugoutput         using  Tracer t   new Tracer  type   Run                  for  int i   0  i  lt  4  i                          DoSomeThingAsync i                       Application Run        Start window message pump to prevent termination     private async void DoSomeThingAsync int i        using  Tracer t   new Tracer  type   DoSomeThingAsync                  t Info  Hi in DoSomething  0   i           try                       int result   await Calculate i               t Info  Got async result   0    result                     catch  ArgumentException ex                        t Error  Got argument exception   0    ex                      Task lt int gt  Calculate int i        var t   new Task lt int gt       gt                using  Tracer t2   new Tracer  type   Calculate                          if  i   2    0                   throw new ArgumentException String Format  Even argument  0    i                return i                          t Start        return t      When you observe the traces   22 25 12 649  02172 02820            AsyncTest Program Run  22 25 12 656  02172 02820            AsyncTest Program DoSomeThingAsync      22 25 12 657  02172 02820 Information AsyncTest Program DoSomeThingAsync Hi in DoSomething 0     22 25 12 658  02172 05220            AsyncTest Program Calculate     22 25 12 659  02172 02820            AsyncTest Program DoSomeThingAsync      22 25 12 659  02172 02820 Information AsyncTest Program DoSomeThingAsync Hi in DoSomething 1     22 25 12 660  02172 02756            AsyncTest Program Calculate     22 25 12 662  02172 02820            AsyncTest Program DoSomeThingAsync      22 25 12 662  02172 02820 Information AsyncTest Program DoSomeThingAsync Hi in DoSomething 2     22 25 12 662  02172 02820            AsyncTest Program DoSomeThingAsync      22 25 12 662  02172 02820 Information AsyncTest Program DoSomeThingAsync Hi in DoSomething 3     22 25 12 664  02172 02756            AsyncTest Program Calculate Duration 4ms    22 25 12 666  02172 02820            AsyncTest Program Run Duration 17ms  ---- Run has completed  The async methods are now scheduled on different threads   22 25 12 667  02172 02756 Information AsyncTest Program DoSomeThingAsync Got async result  1     22 25 12 667  02172 02756            AsyncTest Program DoSomeThingAsync Duration 8ms     22 25 12 667  02172 02756            AsyncTest Program Calculate     22 25 12 665  02172 05220 Exception   AsyncTest Program Calculate Exception thrown  System ArgumentException  Even argument 0       at AsyncTest Program c  DisplayClassf Calculateb  e   in C  Source AsyncTest AsyncTest Program cs line 124       at System Threading Tasks Task 1 InvokeFuture Object futureAsObj         at System Threading Tasks Task InnerInvoke           at System Threading Tasks Task Execute        22 25 12 668  02172 02756 Exception   AsyncTest Program Calculate Exception thrown  System ArgumentException  Even argument 2       at AsyncTest Program c  DisplayClassf Calculateb  e   in C  Source AsyncTest AsyncTest Program cs line 124       at System Threading Tasks Task 1 InvokeFuture Object futureAsObj         at System Threading Tasks Task InnerInvoke           at System Threading Tasks Task Execute        22 25 12 724  02172 05220            AsyncTest Program Calculate Duration 66ms       22 25 12 724  02172 02756            AsyncTest Program Calculate Duration 57ms       22 25 12 725  02172 05220 Error       AsyncTest Program DoSomeThingAsync Got argument exception  System ArgumentException  Even argument 0    Server stack trace          at AsyncTest Program c  DisplayClassf Calculateb  e   in C  Source AsyncTest AsyncTest Program cs line 124       at System Threading Tasks Task 1 InvokeFuture Object futureAsObj         at System Threading Tasks Task InnerInvoke           at System Threading Tasks Task Execute         Exception rethrown at  0            at System Runtime CompilerServices TaskAwaiter EndAwait          at System Runtime CompilerServices TaskAwaiter 1 EndAwait        at AsyncTest Program DoSomeThingAsyncd  8 MoveNext   in C  Source AsyncTest AsyncTest Program cs line 106     22 25 12 725  02172 02756 Error       AsyncTest Program DoSomeThingAsync Got argument exception  System ArgumentException  Even argument 2    Server stack trace          at AsyncTest Program c  DisplayClassf Calculateb  e   in C  Source AsyncTest AsyncTest Program cs line 124       at System Threading Tasks Task 1 InvokeFuture Object futureAsObj         at System Threading Tasks Task InnerInvoke           at System Threading Tasks Task Execute         Exception rethrown at  0            at System Runtime CompilerServices TaskAwaiter EndAwait          at System Runtime CompilerServices TaskAwaiter 1 EndAwait        at AsyncTest Program DoSomeThingAsyncd  8 MoveNext   in C  Source AsyncTest AsyncTest Program cs line 0       22 25 12 726  02172 05220            AsyncTest Program DoSomeThingAsync Duration 70ms    22 25 12 726  02172 02756            AsyncTest Program DoSomeThingAsync Duration 64ms    22 25 12 726  02172 05220            AsyncTest Program Calculate     22 25 12 726  02172 05220            AsyncTest Program Calculate Duration 0ms    22 25 12 726  02172 05220 Information AsyncTest Program DoSomeThingAsync Got async result  3     22 25 12 726  02172 05220            AsyncTest Program DoSomeThingAsync Duration 64ms      You will notice that the Run method completes on thread 2820 while only one child thread has finished  2756   If you put a try catch around your await method you can  catch  the exception in the usual way although your code is executed on another thread when the calculation task has finished and your contiuation is executed    The calculation method traces the thrown exception automatically because I did use the ApiChange Api dll from the ApiChange tool  Tracing and Reflector helps a lot to understand what is going on  To get rid of threading you can create your own versions of GetAwaiter BeginAwait and EndAwait and wrap not a task but e g  a Lazy and trace inside your own extension methods  Then you will get much better understanding what the compiler and what the TPL does   Now you see that there is no way to get in a try catch your exception back since there is no stack frame left for any exception to propagate from  Your code might be doing something totally different after you did initiate the async operations  It might call Thread Sleep or even terminate  As long as there is one foreground thread left your application will happily continue to execute asynchronous tasks      You can handle the exception inside the async method after your asynchronous operation did finish and call back into the UI thread  The recommended way to do this is with TaskScheduler FromSynchronizationContext  That does only work if you have an UI thread and it is not very busy with other things

User · Answer

It s somewhat weird to read but yes  the exception will bubble up to the calling code - but only if you await or Wait   the call to Foo   public async Task Foo         var x   await DoSomethingAsync       public async void DoFoo         try               await Foo              catch  ProtocolException ex                     The exception will be caught because you ve awaited              the call in an async method             or    public void DoFoo         try               Foo   Wait              catch  ProtocolException ex                     The exception will be caught because you ve              waited for the completion of the call                   Async void methods have different error-handling semantics  When an exception is thrown out of an async Task or async Task method  that exception is captured and placed on the Task object  With async void methods  there is no Task object  so any exceptions thrown out of an async void method will be raised directly on the SynchronizationContext that was active when the async void method started  - https   msdn microsoft com en-us magazine jj991977 aspx   Note that using Wait   may cause your application to block  if  Net decides to execute your method synchronously   This explanation http   www interact-sw co uk iangblog 2010 11 01 csharp5-async-exceptions is pretty good - it discusses the steps the compiler takes to achieve this magic

User · Answer

Its also important to note that you will lose the chronological stack trace of the exception if you you have a void return type on an async method  I would recommend returning Task as follows  Going to make debugging a whole lot easier   public async Task DoFoo                 try                       return await Foo                      catch  ProtocolException ex                           Exception with chronological stack trace

User · Answer

The exception can be caught in the async function   public async void Foo         try               var x   await DoSomethingAsync               Handle the result  but sometimes an exception might be thrown            For example  DoSomethingAsync get s data from the network            and the data is invalid    a ProtocolException might be thrown              catch  ProtocolException ex                     The exception will be caught here             public void DoFoo         Foo

User · Answer

This blog explains your problem neatly Async Best Practices   The gist of it being you shouldn t use void as return for an async method  unless it s an async event handler  this is bad practice because it doesn t allow exceptions to be caught  -    Best practice would be to change the return type to Task  Also  try to code async all the way trough  make every async method call and be called from async methods  Except for a Main method in a console  which can t be async  before C  7 1    You will run into deadlocks with GUI and ASP NET applications if you ignore this best practice  The deadlock occurs because these applications runs on a context that allows only one thread and won t relinquish it to the async thread  This means the GUI waits synchronously for a return  while the async method waits for the context  deadlock    This behaviour won t happen in a console application  because it runs on context with a thread pool  The async method will return on another thread which will be scheduled  This is why a test console app will work  but the same calls will deadlock in other applications

[c#] Catch an exception thrown by an async void method

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