Async in C#, the Old and the New

A presentation at DDD North 2020 in February 2020 in Hull, UK by Stuart Lang

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Async in C#, the Old and the New

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HELLO!

I am Stuart Lang You can find me at stu.dev DMs open!

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Agenda

  • Introduction to async and how it works
  • A look at the new guidance - do’s & don’ts
  • Some new stuff
    • Async Streams
    • Channels

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Why async/await

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How does it work?

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How does it work?

Compiler generated code

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How does it work?

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Safe Abstractions Dangerous Abstractions Async/Await “Powerful” Leaky Magic

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“It really troubles me how much async, bad async, really bad async we see in the wild” Kathleen Dollard - .NET Rocks! #1143

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Link 🔗

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Asynchrony is viral - ❌ Bad

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Asynchrony is viral - ✅ Good

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Async void - ❌ Bad

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Async void - ✅ Good

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CancellationTokens - ❌ Bad

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CancellationTokens - ✅ Good

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Using a timeout - ❌ Bad

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Using a timeout - ✅ Good

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FlushAsync & Dispose - ❌ Bad

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FlushAsync & Dispose - ✅ Good

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FlushAsync & Dispose - ✅ Good

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FlushAsync & Dispose - ✅ Good

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async/await vs return Task- ❌ Bad

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async/await vs return Task - ✅ Good

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Prefer Task.FromResult over Task.Run - ❌ Bad

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Prefer Task.FromResult over Task.Run - ✅ Good

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Prefer Task.FromResult over Task.Run - ✅ Good

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TaskCompletionSource - ❌ Bad

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TaskCompletionSource - ✅ Good

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Avoid Task.Run for long blocking - ❌ Bad

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Avoid Task.Run for long blocking - ✅ Good

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Do not use .Wait() or .Result ❌

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SynchronizationContext?

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How does it work?

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How does it work?

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Synchronization Context

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Synchronization Context

WindowsFormsSynchronizationContext DispatcherSynchronizationContext Default (ThreadPool) SynchronizationContext AspNetSynchronizationContext stu.dev

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Synchronization Context

Specific Thread Used to Execute Delegates Exclusive (Delegates Execute One at a Time) WinForms Yes Yes WPF Yes Yes Default No No ASP.NET No Yes

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KA-BLAMO!

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KA-BLAMO!

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ContextFreeTask

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vs-threading

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.ConfigureAwait(false)

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IAsyncEnumerable<T>

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The problem

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Familiar definition

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Familiar definition

IObservable<T> IEnumerable<T> IAsyncEnumerable<T>

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Demo - IAsyncEnumerable<T> & Channels

Watch the demo here!

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THANKS!

Any questions? You can find me at: stu.dev