async await - when to return a Task vs void

Question

Under what scenarios would one want to use   public async Task AsyncMethod int num    instead of   public async void AsyncMethod int num    The only scenario that I can think of is if you need the task to be able to track its progress    Additionally  in the following method  are the async and await keywords unnecessary   public static async void AsyncMethod2 int num        await Task Factory StartNew      gt  Thread Sleep num

User · Answer

The problem with calling async void is that

you don’t even get the task back. You have no way of knowing when the function’s task has completed. —— Crash course in async and await | The Old New Thing

Here are the three ways to call an async function:

async Task<T> SomethingAsync() { ... return t; }
async Task SomethingAsync() { ... }
async void SomethingAsync() { ... }
In all the cases, the function is transformed into a chain of tasks. The difference is what the function returns.

In the first case, the function returns a task that eventually produces the t.

In the second case, the function returns a task which has no product, but you can still await on it to know when it has run to completion.

The third case is the nasty one. The third case is like the second case, except that you don't even get the task back. You have no way of knowing when the function's task has completed.

The async void case is a "fire and forget": You start the task chain, but you don't care about when it's finished. When the function returns, all you know is that everything up to the first await has executed. Everything after the first await will run at some unspecified point in the future that you have no access to.

User · Answer

I have come across this very useful article about async and void written by J  r  me Laban  https   jaylee org archive 2012 07 08 c-sharp-async-tips-and-tricks-part-2-async-void html The bottom line is that an async void can crash the system and usually should be used only on the UI side event handlers   The reason behind this is the Synchronization Context used by the AsyncVoidMethodBuilder  being none in this example  When there is no ambient Synchronization Context  any exception that is unhandled by the body of an async void method is rethrown on the ThreadPool  While there is seemingly no other logical place where that kind of unhandled exception could be thrown  the unfortunate effect is that the process is being terminated  because unhandled exceptions on the ThreadPool effectively terminate the process since  NET 2 0  You may intercept all unhandled exception using the AppDomain UnhandledException event  but there is no way to recover the process from this event  When writing UI event handlers  async void methods are somehow painless because exceptions are treated the same way found in non-async methods  they are thrown on the Dispatcher  There is a possibility to recover from such exceptions  with is more than correct for most cases  Outside of UI event handlers however  async void methods are somehow dangerous to use and may not that easy to find

User · Answer

I got clear idea from this statements    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 SynchronizationContext represents a location  where  code might be executed    that was active when the async void method started   Exceptions from an Async Void Method Can   t Be Caught with Catch  private async void ThrowExceptionAsync       throw new InvalidOperationException      public void AsyncVoidExceptions CannotBeCaughtByCatch       try         ThrowExceptionAsync          catch  Exception             The exception is never caught here      throw          These exceptions can be observed using AppDomain UnhandledException or a similar catch-all event for GUI ASP NET applications  but using those events for regular exception handling is a recipe for unmaintainability it crashes the application     Async void methods have different composing semantics  Async methods returning Task or Task can be easily composed using await  Task WhenAny  Task WhenAll and so on  Async methods returning void don   t provide an easy way to notify the calling code that they   ve completed  It   s easy to start several async void methods  but it   s not easy to determine when they   ve finished  Async void methods will notify their SynchronizationContext when they start and finish  but a custom SynchronizationContext is a complex solution for regular application code  Async Void method useful when using synchronous event handler because they raise their exceptions directly on the SynchronizationContext  which is similar to how synchronous event handlers behave   For more details check this link https   msdn microsoft com en-us magazine jj991977 aspx

User · Answer

I think you can use async void for kicking off background operations as well  so long as you re careful to catch exceptions  Thoughts   class Program        static bool isFinished   false       static void Main string   args                Kick off the background operation and don t care about when it completes         BackgroundWork             Console WriteLine  Press enter when you re ready to stop the background operation             Console ReadLine            isFinished   true                Using async void to kickoff a background operation that nobody wants to be notified about when it completes      static async void BackgroundWork                It s important to catch exceptions so we don t crash the appliation          try                  This operation will end after ten interations or when the app closes  Whichever happens first              for  var count   1  count  lt   10  amp  amp   isFinished  count                      await Task Delay 1000                   Console WriteLine    count  seconds of work elapsed                               Console WriteLine  Background operation came to an end               catch  Exception x                Console WriteLine  Caught exception                 Console WriteLine x ToString

User · Answer

According to Microsoft documentation  should NEVER use async void  Do not do this  The following example uses async void which makes the HTTP request complete when the first await is reached   Which is ALWAYS a bad practice in ASP NET Core apps   Accesses the HttpResponse after the HTTP request is complete   Crashes the process

User · Answer

My answer is simple you can not await void method   Error   CS4008  Cannot await  void  TestAsync   e  test TestAsync TestAsyncProgram cs   So if the method is async it is better to be awaitable  because you can loose async advantage

User · Answer

1  Normally  you would want to return a Task  The main exception should be when you need to have a void return type  for events   If there s no reason to disallow having the caller await your task  why disallow it   2  async methods that return void are special in another aspect  they represent top-level async operations  and have additional rules that come into play when your task returns an exception  The easiest way is to show the difference is with an example   static async void f         await h       static async Task g         await h       static async Task h         throw new NotImplementedException       private void button1 Click object sender  EventArgs e        f       private void button2 Click object sender  EventArgs e        g       private void button3 Click object sender  EventArgs e        GC Collect        f s exception is always  observed   An exception that leaves a top-level asynchronous method is simply treated like any other unhandled exception  g s exception is never observed  When the garbage collector comes to clean up the task  it sees that the task resulted in an exception  and nobody handled the exception  When that happens  the TaskScheduler UnobservedTaskException handler runs  You should never let this happen  To use your example   public static async void AsyncMethod2 int num        await Task Factory StartNew      gt  Thread Sleep num        Yes  use async and await here  they make sure your method still works correctly if an exception is thrown   for more information see  http   msdn microsoft com en-us magazine jj991977 aspx

[c#] async/await - when to return a Task vs void?

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