How to safely call an async method in C without await

Question

I have an async method which returns no data  public async Task MyAsyncMethod            do some stuff async  don t return any data    I m calling this from another method which returns some data  public string GetStringData         MyAsyncMethod       this generates a warning and swallows exceptions     return  quot hello world quot      Calling MyAsyncMethod   without awaiting it causes a  quot Because this call is not awaited  the current method continues to run before the call is completed quot  warning in visual studio  On the page for that warning it states   You should consider suppressing the warning only if you re sure that you don t want to wait for the asynchronous call to complete and that the called method won t raise any exceptions   I m sure I don t want to wait for the call to complete  I don t need to or have the time to  But the call might raise exceptions  I ve stumbled into this problem a few times and I m sure it s a common problem which must have a common solution  How do I safely call an async method without awaiting the result  Update  For people suggesting that I just await the result  this is code that is responding to a web request on our web service  ASP NET Web API   Awaiting in a UI context keeps the UI thread free  but awaiting in a web request call will wait for the Task to finish before responding to the request  thereby increasing response times with no reason

User · Answer

I m late to the party here  but there s an awesome library I ve been using which I haven t seen referenced in the other answers    https   github com brminnick AsyncAwaitBestPractices  If you need to  Fire And Forget  you call the extension method on the task   Passing the action onException to the call ensures that you get the best of both worlds - no need to await execution and slow your users down  whilst retaining the ability to handle the exception in a graceful manner   In your example you would use it like this      public string GetStringData                 MyAsyncMethod   SafeFireAndForget onException   exception    gt                                                DO STUFF WITH THE EXCEPTION                                                      return  hello world           It also gives awaitable AsyncCommands implementing ICommand out the box which is great for my MVVM Xamarin solution

User · Answer

I end up with this solution    public async Task MyAsyncMethod            do some stuff async  don t return any data    public string GetStringData            Run async  no warning  exception are catched     RunAsync MyAsyncMethod          return  hello world      private void RunAsync Task task        task ContinueWith t   gt                ILog log   ServiceLocator Current GetInstance lt ILog gt             log Error  Unexpected Error   t Exception           TaskContinuationOptions OnlyOnFaulted

User · Answer

You should first consider making GetStringData an async method and have it await the task returned from MyAsyncMethod   If you re absolutely sure that you don t need to handle exceptions from MyAsyncMethod or know when it completes  then you can do this   public string GetStringData       var     MyAsyncMethod      return  hello world       BTW  this is not a  common problem   It s very rare to want to execute some code and not care whether it completes and not care whether it completes successfully   Update   Since you re on ASP NET and wanting to return early  you may find my blog post on the subject useful  However  ASP NET was not designed for this  and there s no guarantee that your code will run after the response is returned  ASP NET will do its best to let it run  but it can t guarantee it   So  this is a fine solution for something simple like tossing an event into a log where it doesn t really matter if you lose a few here and there  It s not a good solution for any kind of business-critical operations  In those situations  you must adopt a more complex architecture  with a persistent way to save the operations  e g   Azure Queues  MSMQ  and a separate background process  e g   Azure Worker Role  Win32 Service  to process them

User · Answer

The solution is start the HttpClient into another execution task without sincronization context    var submit   httpClient PostAsync uri  new StringContent body  Encoding UTF8  application json     var t   Task Run      gt  submit ConfigureAwait false    await t ConfigureAwait false

User · Answer

If you want to get the exception  quot asynchronously quot   you could do    MyAsyncMethod        ContinueWith t   gt  Console WriteLine t Exception           TaskContinuationOptions OnlyOnFaulted    This will allow you to deal with an exception on a thread other than the  quot main quot  thread   This means you don t have to  quot wait quot  for the call to MyAsyncMethod   from the thread that calls MyAsyncMethod  but  still allows you  to do something with an exception--but only if an exception occurs  Update  technically  you could do something similar with await  try       await MyAsyncMethod   ConfigureAwait false     catch  Exception ex        Trace WriteLine ex         which would be useful if you needed to specifically use try catch  or using  but I find the ContinueWith to be a little more explicit because you have to know what ConfigureAwait false  means

User · Answer

Maybe I m too naive but  couldn t you create an event that is raised when GetStringData   is called and attach an EventHandler that calls and awaits the async method  Something like  public event EventHandler FireAsync   public string GetStringData        FireAsync  Invoke this  EventArgs Empty      return  quot hello world quot      public async void HandleFireAsync object sender  EventArgs e       await MyAsyncMethod       And somewhere in the code attach and detach from the event  FireAsync    HandleFireAsync          FireAsync -  HandleFireAsync   Not sure if this might be anti-pattern somehow  if it is please let me know   but it catches the Exceptions and returns quickly from GetStringData

User · Answer

If you really wants to do this  Just to address  Call an async method in C  without await   you can execute the async method inside a Task Run  This approach will wait until MyAsyncMethod finish   public string GetStringData         Task Run     gt  MyAsyncMethod    Result      return  hello world       await asynchronously unwraps the Result of your task  whereas just using Result would block until the task had completed

User · Answer

I guess the question arises  why would you need to do this  The reason for async in C  5 0 is so you can await a result  This method is not actually asynchronous  but simply called at a time so as not to interfere too much with the current thread   Perhaps it may be better to start a thread and leave it to finish on its own

User · Answer

This is called fire and forget  and there is an extension for that      Consumes a task and doesn t do anything with it  Useful for fire-and-forget calls to async methods within async methods    Install nuget package   Use   MyAsyncMethod   Forget

User · Answer

The answer by Peter Ritchie was what I wanted  and Stephen Cleary s article about returning early in ASP NET was very helpful   As a more general problem however  not specific to an ASP NET context  the following Console application demonstrates the usage and behavior of Peter s answer using Task ContinueWith       static void Main string   args      try            output  hello world  as method returns early     Console WriteLine GetStringData           catch            Exception is NOT caught here       Console ReadLine       public static string GetStringData       MyAsyncMethod   ContinueWith OnMyAsyncMethodFailed  TaskContinuationOptions OnlyOnFaulted     return  hello world      public static async Task MyAsyncMethod       await Task Run      gt    throw new Exception  thrown on background thread           public static void OnMyAsyncMethodFailed Task task      Exception ex   task Exception       Deal with exceptions here however you want     GetStringData   returns early without awaiting MyAsyncMethod   and exceptions thrown in MyAsyncMethod   are dealt with in OnMyAsyncMethodFailed Task task  and not in the try catch around GetStringData

User · Answer

On technologies with message loops  not sure if ASP is one of them   you can block the loop and process messages until the task is over  and use ContinueWith to unblock the code   public void WaitForTask Task task        DispatcherFrame frame   new DispatcherFrame        task ContinueWith t   gt  frame Continue   false        Dispatcher PushFrame frame       This approach is similar to blocking on ShowDialog and still keeping the UI responsive

User · Answer

Typically async method returns Task class  If you use Wait   method or Result property and code throws exception - exception type gets wrapped up into AggregateException - then you need to query Exception InnerException to locate correct exception   But it s also possible to use  GetAwaiter   GetResult   instead - it will also wait async task  but will not wrap exception   So here is short example   public async Task MyMethodAsync        public string GetStringData         MyMethodAsync   GetAwaiter   GetResult        return  test       You might want also to be able to return some parameter from async function - that can be achieved by providing extra Action lt return type gt  into async function  for example like this   public string GetStringData         return MyMethodWithReturnParameterAsync   GetAwaiter   GetResult       public async Task lt String gt  MyMethodWithReturnParameterAsync         return  test       Please note that async methods typically have ASync suffix naming  just to be able to avoid collision between sync functions with same name   E g  FileStream ReadAsync  - I have updated function names to follow this recommendation

[c#] How to safely call an async method in C# without await

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