How to wait for async method to complete

Question

I m writing a WinForms application that transfers data to a USB HID class device   My application uses the excellent Generic HID library v6 0 which can be found here   In a nutshell  when I need to write data to the device  this is the code that gets called   private async void RequestToSendOutputReport List lt byte   gt  byteArrays        foreach  byte   b in byteArrays                while  condition                           we ll typically execute this code many times until the condition is no longer met             Task t   SendOutputReportViaInterruptTransfer                await t                        read some data from device  we need to wait for this to return         RequestToGetInputReport              When my code drops out of the while loop  I need to read some data from the device   However  the device isn t able to respond right away so I need to wait for this call to return before I continue   As it currently exists  RequestToGetInputReport   is declared like this   private async void RequestToGetInputReport            lots of code prior to this     int bytesRead   await GetInputReportViaInterruptTransfer        For what it s worth  the declaration for GetInputReportViaInterruptTransfer   looks like this   internal async Task lt int gt  GetInputReportViaInterruptTransfer     Unfortunately  I m not very familiar with the workings of the new async await technologies in  NET 4 5   I did a little reading earlier about the await keyword and that gave me the impression that the call to GetInputReportViaInterruptTransfer   inside of RequestToGetInputReport   would wait  and maybe it does   but it doesn t seem like the call to RequestToGetInputReport   itself is waiting because I seem to be re-entering the while loop almost immediately   Can anyone clarify the behavior that I m seeing

User · Answer

The following snippet shows a way to ensure the awaited method completes before returning to the caller  HOWEVER  I wouldn t say it s good practice  Please edit my answer with explanations if you think otherwise   public async Task AnAsyncMethodThatCompletes         await SomeAsyncMethod        DoSomeMoreStuff        await Task Factory StartNew      gt            lt -- This line here  at the end    await AnAsyncMethodThatCompletes    Console WriteLine  AnAsyncMethodThatCompletes   completed

User · Answer

The most important thing to know about async and await is that await doesn t wait for the associated call to complete  What await does is to return the result of the operation immediately and synchronously if the operation has already completed or  if it hasn t  to schedule a continuation to execute the remainder of the async method and then to return control to the caller  When the asynchronous operation completes  the scheduled completion will then execute   The answer to the specific question in your question s title is to block on an async method s return value  which should be of type Task or Task lt T gt   by calling an appropriate Wait method   public static async Task lt Foo gt  GetFooAsync            Start asynchronous operation s  and return associated task             public static Foo CallGetFooAsyncAndWaitOnResult         var task   GetFooAsync        task Wait       Blocks current thread until GetFooAsync task completes                     For pedagogical use only  in general  don t do this      var result   task Result      return result      In this code snippet  CallGetFooAsyncAndWaitOnResult is a synchronous wrapper around asynchronous method GetFooAsync  However  this pattern is to be avoided for the most part since it will block a whole thread pool thread for the duration of the asynchronous operation  This an inefficient use of the various asynchronous mechanisms exposed by APIs that go to great efforts to provide them   The answer at  quot await quot  doesn  39 t wait for the completion of call has several  more detailed  explanations of these keywords   Meanwhile   Stephen Cleary s guidance about async void holds  Other nice explanations for why can be found at http   www tonicodes net blog why-you-should-almost-never-write-void-asynchronous-methods  and https   jaylee org archive 2012 07 08 c-sharp-async-tips-and-tricks-part-2-async-void html

User · Answer

Here is a workaround using a flag     outside your event or method  but inside your class private bool IsExecuted   false   private async Task MethodA        Do Stuff Here  IsExecuted   true              Inside your event or method    await MethodA     while   isExecuted  Thread Sleep 200       lt -------  await MethodB

User · Answer

Actually I found this more helpful for functions that return IAsyncAction               var task   asyncFunction                while  task Status    AsyncStatus Completed

User · Answer

just put Wait   to wait until task completed  GetInputReportViaInterruptTransfer   Wait

User · Answer

Avoid async void  Have your methods return Task instead of void  Then you can await them   Like this   private async Task RequestToSendOutputReport List lt byte   gt  byteArrays        foreach  byte   b in byteArrays                while  condition                           we ll typically execute this code many times until the condition is no longer met             Task t   SendOutputReportViaInterruptTransfer                await t                        read some data from device  we need to wait for this to return         await RequestToGetInputReport             private async Task RequestToGetInputReport            lots of code prior to this     int bytesRead   await GetInputReportViaInterruptTransfer

User · Answer

Best Solution to wait AsynMethod till complete the task is   var result   Task Run async     gt  await yourAsyncMethod    Result

[c#] How to wait for async method to complete?

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