Correcting Common Async Await Mistakes in .NET

Did you know that the .NET compiler turns our async methods into classes? And that .NET adds a try/catch block to each of these classes, potentially hiding thrown exceptions? It's true!

In this session, we will learn how to best use async/await in C# by analyzing how .NET compiles our async code.

Join me as we take an existing app and optimize its async code together, showing off performance gains, better exception handling, improved run-time speed, smaller app size and more using the latest tools in C#12 + .NET 8!

1. Correcting Common Async/Await Mistakes Brandon Minnick .NET Developer Advocate

2. ReadDataFromUrl async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); }

3. ReadDataFromUrl async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } Thread 1

4. ReadDataFromUrl async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } Thread 2* *Can be any thread other than Thread 1 e.g. Thread 32

5. ReadDataFromUrl async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } Thread 1

6. Compiler Generated Code

7. async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } ReadDataFromUrl

8. async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } ReadDataFromUrl private sealed class < ReadDataFromUrl >d_1 : IAsyncStateMachine

9. async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } ReadDataFromUrl private string <data>5_3; private byte[] <result>5_2; private WebClient < wc >5_1; public string url ;

10. async Task   ReadDataFromUrl ( string url ) { WebClient   wc  =  new   WebClient ();     byte[]  result =  await   wc.DownloadDataTaskAsync ( url );     string  data =  Encoding . ASCII .GetString (result);     LoadData (data); } ReadDataFromUrl private void MoveNext ();

11. ReadDataFromUrl_MoveNext public   void   MoveNext () { uint   num  = ( uint ) this .$PC;     this .$PC = -1;     try  {        switch  ( num ) {       case  0:            this .< wc >__0 =  new   WebClient ();              this .$awaiter0 =  this .< wc >__0.DownloadDataTaskAsync( this .url ). GetAwaiter ();             this .$PC = 1; ...              return ;              break ;          case  1:              this .< result >__1 =  this .$awaiter0.GetResult();              this .<data>__2 =  Encoding.ASCII.GetString ( this .< result >__1);              this .$ this.LoadData ( this .<data>__2);              break ;           default :              return ;     }    }     catch  ( Exception   exception ) { ... }     this .$PC = -1;     this .$ builder.SetResult (); }

12. public   void   MoveNext () { uint   num  = ( uint ) this .$PC;     this .$PC = -1;     try  {        switch  ( num ) {       case  0:            this .< wc >__0 =  new   WebClient ();              this .$awaiter0 =  this .< wc >__0.DownloadDataTaskAsync( this .url ). GetAwaiter ();             this .$PC = 1; ...              return ;              break ;          case  1:              this .< result >__1 =  this .$awaiter0.GetResult();              this .<data>__2 =  Encoding.ASCII.GetString ( this .< result >__1);              this .$ this.LoadData ( this .<data>__2);              break ;           default :              return ;     }    }     catch  ( Exception   exception ) { ... }     this .$PC = -1;     this .$ builder.SetResult (); } ReadDataFromUrl_MoveNext case  0: this .< wc >__0 =  new   WebClient (); this .$awaiter0 =  this .< wc >__0.DownloadDataTaskAsync( this .url ). GetAwaiter (); this .$PC = 1; ... return ;

13. public   void   MoveNext () { uint   num  = ( uint ) this .$PC;     this .$PC = -1;     try  {        switch  ( num ) {       case  0:            this .< wc >__0 =  new   WebClient ();              this .$awaiter0 =  this .< wc >__0.DownloadDataTaskAsync( this .url ). GetAwaiter ();             this .$PC = 1; ...              return ;              break ;          case  1:              this .< result >__1 =  this .$awaiter0.GetResult();              this .<data>__2 =  Encoding.ASCII.GetString ( this .< result >__1);              this .$ this.LoadData ( this .<data>__2);              break ;           default :              return ;     }    }     catch  ( Exception   exception ) { ... }     this .$PC = -1;     this .$ builder.SetResult (); } ReadDataFromUrl_MoveNext case  1: this .< result >__1 =  this .$awaiter0.GetResult(); this .<data>__2 =  Encoding.ASCII.GetString ( this .< result >__1); this .$ this.LoadData ( this .<data>__2); break ;

14. public   void   MoveNext () { uint   num  = ( uint ) this .$PC;     this .$PC = -1;     try  {        switch  ( num ) {       case  0:            this .< wc >__0 =  new   WebClient ();              this .$awaiter0 =  this .< wc >__0.DownloadDataTaskAsync( this .url ). GetAwaiter ();             this .$PC = 1; ...              return ;              break ;          case  1:              this .< result >__1 =  this .$awaiter0.GetResult();              this .<data>__2 =  Encoding.ASCII.GetString ( this .< result >__1);              this .$ this.LoadData ( this .<data>__2);              break ;           default :              return ;     }    }     catch  ( Exception   exception ) { ... }     this .$PC = -1;     this .$ builder.SetResult (); } ReadDataFromUrl_MoveNext try { catch ( Exception exception ) { . . . }

15. Quick Review

16. Async Keyword Adds 100 Bytes Every Async Method Becomes a Class

17. Await Every Task Non-awaited Tasks Hide Exceptions

18. Let’s Fix Some Code

19. Async/Await Best Practices

20. Async/Await Best Practices Never Use ` .Wait()` or ` .Result` Always use ` await` If synchronous, use ` . GetAwaiter (). GetResult ()`

21. Async/Await Best Practices Fire and Forget Tasks Use ` SafeFireAndForget ` NuGet: AsyncAwaitBestPractices

22. Async/Await Best Practices Avoid ` return await` Remove ` async` keyword Except: In ` try/catch` blocks Except: In ` using( … )` blocks

23. Async/Await Best Practices Utilize `. ConfigureAwait (false)` Except: When needing to return to calling thread Note: Only works when framework is using SynchronizationContext WPF WinForms Xamarin .NET MAUI WinUI ASP.NET Core

24. Async/Await Best Practices Utilize ` ValueTask ` When a method’s “hot path” doesn’t call ` await `

25. Async/Await Best Practices ` IAsyncEnumerable ` for Streaming Data Allows us to update UI as data arrives Better User Experience Use ` [ EnumeratorCancellation ] ` for CancellationToken

26. Async/Await Best Practices ` . WaitAsync ( CancellationToken )` Append to any async method missing a ` CancellationToken ` parameter

27. Async/Await Best Practices ` IAsyncDisposable ` The “await” executes at the end of the ` using ` block Can Append ConfigureAwait (false) CancellationToken not currently supported

28. https:// codetraveler.io / AsyncAwaitBestPractices / Resources

29. Thank You https:// codetraveler.io / AsyncAwaitBestPractices /

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