C# async ⚡️ await
is not your playground

#DevUgZg, Zagreb 2019-11-27
http://bit.ly/DevUgZg-async-await

Vedran Mandić, MCSD, MCSA, MCT

Hi, I am Vedran. 👋

I freelance, talk and lecture C# / .NET and JavaScript
currently in insurance industry
on medium and dev.to by @vekzdran
on github by @vmandic
in this since 2009, fresh!
bad at basics, suck at CSS, moderate in C#... jk 🙈

Plan?

What is so hard with (C#) async code?
Must-know 🌟 terminology, (PRO)TIPS!
async / await FAQ 10+ examples
- demo code 👩‍💻
Q&A (please after 3) ⏰⏰⏰
Study, share and apply, please...

30min + 30min (very fast tempo)

Why is async...

...code hard to grasp?

System.AggregateException: One or more errors occurred. ---> System.AggregateException: One or more errors occurred. ---> System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 29
   --- End of inner exception stack trace ---
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 39
   --- End of inner exception stack trace ---
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 51
---> (Inner Exception #0) System.AggregateException: One or more errors occurred. ---> System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 29
   --- End of inner exception stack trace ---
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 39
---> (Inner Exception #0) System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 29<---

---> (Inner Exception #1) System.FormatException: One of the identified items was in an invalid format.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 35<---
<---

---> (Inner Exception #1) System.NotImplementedException: The method or operation is not implemented.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 47<---
System.AggregateException: One or more errors occurred. ---> System.NotImplementedException: The method or operation is not implemented.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 47
   --- End of inner exception stack trace ---
---> (Inner Exception #0) System.NotImplementedException: The method or operation is not implemented.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 47<---

---> (Inner Exception #1) System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 29<---

---> (Inner Exception #2) System.FormatException: One of the identified items was in an invalid format.
   at ConsoleApplication6.Program.Main(String[] args) in C:\Projects\ConsoleApplication6\ConsoleApplication6\Program.cs:line 35<---

understanding parallel vs async
APM, EAP, BackgroundWorker, Parallel, TPL or async / await !?
"types" and "roles" of threads ?
how / why to create a new Thread() ?
how / why to run a new Task() ? Should I use .ContinueWith() ?
what is a SynchronizationContext ?
what is ThreadPool and TaskScheduler, how do they execute ?
what is Thread synchronization or joining ?
what are locks, semaphors and monitors ?
why does my Windows app crash from another thread ?
difference or link between Thread and Task ?
when to use async / await syntax, why avoid async void ?
why are Task exceptions swollen, why do I get Deadlocks ?
when to use Task or ValueTask, what is IAsyncEnumerable !?
what is .ConfigureAwait(false) and WHY should(n't) I use it ?
what happend in .NET Core with SynchronizationContext !?!?
what is "sync over async" and why you should not do it ?
SHOULD I USE ASYNC ALAWAYS ?
IS IT FASTER OR BETTER !?

📸

This is why
C# async code is hard...

Some...
(raged but on point) community feedback on C# async design

https://developercommunity.visualstudio.com/idea/583945/improve-asynchronous-programming-model.html

async void: I wish we could have not allowed that...

- Mads Torgersen [MSFT], Oct 24 2019

(very boring) Important

Terminology Recap

You can take a sip of 🍺 now...

System.Threading.Thread

handles CPU time to process (your app's) work
has priority, (synchronization) context, state
an OS thread is mapped to a managed .NET Thread
different roles e.g. main, UI, worker, ThreadPool
different types e.g. foreground and background
new Thread() is not from a ThreadPool & has no SyncContext
can run in Multiple/Parallel
synced by many constructs such as lock() { ... }, monitors...
TIP: favor ThreadPool threads to save memory and time

System.Threading.ThreadPool

your .NET app (AppDomain) gets / has one
"Worker" and "I/O Completion Port" threads
reserve of (app process) worker threads
has a min and max limit, can be changed, per CPU
exceeding min limit throttles for 500ms (Redis anyone!?)
careful not to exhaust and spike memory
ThreadPool bg threads won't block the fg threads
[ThreadStaticAttribute] values are not cleared on reuse

TIP: How to manage ThreadPool minimum and throttling

System.Threading.Tasks.Task

thread 1

thread 42

thread 3

thread 114

thread 3

thread 1

System.Threading.Tasks.Task

from .NET v4.0, before async / await
promise of work to be done (by a Thread)
uses (by default) bg Threads from ThreadPool
TIP: use LongRunning Tasks if action takes long
can be awaited (in an async scope)
Task<T> to return a T .Result (else void .Wait())
can be nested / attached
TIP: use .Unwrap() to resolve inner Task<Task<T>> results

System.Threading.SynchronizationContext

System.Threading.SynchronizationContext 1/2

represents a "location" where || how a Thread is run
WinFormsSynchronizationContext, AspNetSC, default...
behaviour differs across .NET app type impl.
designed to send data across threads
WinForms forbids updating UI from other contexts!
await captures the current context and resumes on it
ASP.NET Core has no SynchronizationContext:
-> perf is inherently better!

TIP: Understand the need for SyncContext by S. Cleary,
MSDN Feb 2011

System.Threading.SynchronizationContext 2/2

a null SynchronizationContext of a Thread is a new SynchronizationContext(), unrestricted, with access to the ThreadPool
WinForms, WPF, ASP.NET Framework have synchronizing SynchronizationContexts:
- you can execute only one task after each other per active thread...
BEWARE: Upgrading from full .NET ASP.NET framework to ASP.NET Core may cause implicit parallelism and race condition issues

TIP: Why ASP.NET Core has no SyncContext, S. Cleary

TaskScheduler

of System.Threading.Tasks

TaskScheduler

System.Threading.Tasks

handles app Tasks scheduling & execution
Abstraction over SynchronizationContext
can get current SyncContext
- TaskScheduler.FromCurrentSynchronizationContext();
"queues Tasks onto Threads" MSDN
TaskScheduler.Default is the default impl.
->ThreadPool global & local queues (parent vs child)
abstract but inheritable
inherit your own, e.g. limit number of Threads in app

source: https://medium.com/rubrikkgroup/understanding-async-avoiding-deadlocks-e41f8f2c6f5d

How is a task scheduled?

Deadlock

one or more threads block / wait / hang on a line of code indefinitely, waiting for another thread to finish,
the app is blocked and UNUSABLE from that point
e.g. WinForms (SynchronizationContext) misuse when updating the MainForm UI controls from non MainForm threads:
e.g. ASP.NET (non .NET Core) accessing .Result property or .Wait() method of non-awaited Task

 Task.Run(() => lblInfo.Text = "Boom!");

(thread) Blocking

blocked thread waits / hangs and can not be used for other work execution, process memory usage spikes if you keep spawning them in this scenario, ThreadPool exhaustion is imminent, e.g. an endpoint calling .Result
applying valid async / await syntax releases the awaiting thread back to the ThreadPool and respects SynchronizationContext implementations

some e-z win tips...

PROTIP 1:

use async / await (all the way) if unsure or must have

no need for overengineering with Parallel or Threads

public async Task<string> GetUniqueUsernameAsync(int id) 
{
    var user = await db.Users.SingleOrDefaultAsync(x => x.Id == id);
    return user?.Username ?? throw new UserNotFoundException(id);
}

PROTIP 2:

please meassure perf!

async / await is not always good!

but... e.g. do apply it on a highly requested web endpoint

public string GetUniqueUsername(int id) 
{
    var user = db.Users.SingleOrDefault(x => x.Id == id);
    return user?.Username ?? throw new UserNotFoundException(id);
}

PROTIP 3:

avoid if possible dangerous "sync over async" blocking

good luck catching that exception or awaitng the method

public Task<string> GetUniqueUsername(int id) 
{
    // DON'T DO THIS, PLEASE... JUST DON'T
    var user = db.Users.SingleOrDefaultAsync(x => x.Id == id).Result;
    
    return Task.FromResult(user?.Username 
    	?? throw new UserNotFoundException(id));
}

PROTIP 4:

if you must do "sync over async"...

...then at least you'll get a proper exception

public string GetUniqueUsername(int id) 
{
    var user = db.Users.SingleOrDefaultAsync(x => x.Id == id)
    	.GetAwaiter()
    	.GetResult();
    return user?.Username ?? throw new UserNotFoundException(id);
}

Sync

VS Async VS

Parallel

Sync

Crack egg 1
Fry egg 1
Prepare plate 1
Serve egg 1
Crack egg 2
Fry egg 2
Prepare plate 2
Serve egg 2
Crack egg 3
Fry egg 3
Prepare plate 3
Serve egg 3

VS Async VS

Crack and mix 3 eggs
Let fry 3 eggs together
While frying prepare 3 plates
Serve 3 eggs

Parallel

3 cooks crack their egg
3 cooks fry their egg
3 cooks get their plate
3 cooks serve their egg

That's it for the boooooring part... 🔨

demo:
https://github.com/vmandic/you-still-dont-know-async-await

thanks you can leave now for pizza :)

If this was too little... :-)

https://michaelscodingspot.com/2019/01/17/c-deadlocks-in-depth-part-1/
https://michaelscodingspot.com/2019/01/24/c-deadlocks-in-depth-part-2/

Locking and Deadlocks explained by example with async / await

https://odetocode.com/blogs/scott/archive/2019/03/04/await-the-async-letdown.asp

thoughts by a senior C# developer and author on why working with async / await is hard and prone to errors

https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AspNetCoreGuidance.md#aspnet-core-guidance

an excellent summary of multiple how-to and don’ts with async / await in dotnet core

https://dev.to/stuartblang/miscellaneous-c-async-tips-3o47

how to deal with lazy initialization in CTOR that has to become async (AsyncLazy example)

http://blog.i3arnon.com/2015/07/02/task-run-long-running/

TaskCreationOptions.LongRunning explained, one can read that an actual thread (not from the thread pool) is created

http://blog.stephencleary.com/2017/03/aspnetcore-synchronization-context.html - SyncContext in ASP.NET Core and dangers one can face

https://msdn.microsoft.com/en-us/magazine/dn802603.aspx

a detailed overview how async applies to the ASP.NET runtime, how thread-pool allocates threads and when does context switching happen

http://blog.stephencleary.com/2012/07/dont-block-on-async-code.html

the evergreen example of why not to block async calls with .Result or .Wait() in traditional environments which have a dedicated syncContext

https://msdn.microsoft.com/en-us/magazine/gg598924.aspx

why we need the synchronization context

https://markheath.net/post/async-antipatterns

a summary of all the examples and more, fresh 2019 article

https://www.youtube.com/watch?v=av5YNd4X3dY

correcting Common Mistakes When Using Async/Await in .NET - Brandon Minnick

https://www.youtube.com/watch?v=Al8LrBKpZEU

Back to Basics: Efficient Async and Await - Filip Ekberg

https://dev.to/hardkoded/a-fairy-tale-about-async-voids-events-and-error-handling-1afi

real world example of async void impact

https://www.youtube.com/watch?v=-cJjnVTJ5og

“Why your ASP.NET Core application won’t scale?”, has a lot of cool async trap examples at the end - D. Edwards and D. Fowler at NDC London 2019

https://www.youtube.com/watch?v=ghDS4_NFbcQ

“The promise of an async future awaits” - overview of async and await and it’s issues, Bill Wagner (.NET docs team) at NDC London 2019, explaining Sync, Async and Parallel with making an English breakfast example.

https://www.youtube.com/watch?v=XpgN7y_EXps

“Async & Await (You’re doing it wrong)”, good examples of bad usage: async void, async void lambda, async constructor and property, running async in sync (blocking examples)

https://chrisstclair.co.uk/demystifying-async-await

a quick and simple example of how bad application blocks the UI and how to quickly fix it.

https://medium.com/rubrikkgroup/understanding-async-avoiding-deadlocks-e41f8f2c6f5d

when and how can a deadlock occur with async / await, a table view breakdown depending in which execution context the task is run, excellent overview

https://olitee.com/2015/01/c-async-await-common-deadlock-scenario/

a deadlock scenario explained

https://devblogs.microsoft.com/pfxteam/asyncawait-faq/

probably the best and concrete FAQ on async / await there is on the internet

https://devblogs.microsoft.com/pfxteam/await-and-ui-and-deadlocks-oh-my/

the most simple and right explanation of deadlock common scenario when the UI thread is blocked due to .Result or Wait() and can not restore new state from the updated SynchronizationContext (i.e. windows message loop when working with Windows Forms and WPF).

Takeaways

do not overengingeer with Tasks, code review with care
measure perf, don't always assume asnyc / await!
go async all the way from the start to the end
beware of SynchronizationContext and deadlocks
Tweak the ThreadPool if (really...) needed
Measure all cases, memory and time
Do not spawn Task.Run() if not utterly needed
try-catch in async void, legitimate in event handlers
use async APIs: Task.Delay(), DbSet<T>.FirstAsync()...
async saves memory if not abused, else the opposite!!!!
https://github.com/davidfowl/AspNetCoreDiagnosticScenarios

The end! 📸 Questions?

VEDRAN MANDIĆ

mandic.vedran@gmail.com

@vekzdran @vmandic

http://bit.ly/DevUgZg-async-await

Async await is not a playground

By Vedran Mandić

Async await is not a playground

Still feeling unsure when you write an async method in C# with .NET? Well, I got you covered, example by example where you might've failed. This presentation will cover basic terminology and the common async / await pitfalls which might cost you your job.

1,074

Vedran Mandić

functor-software.hr

C# async ⚡️ await is not your playground

Hi, I am Vedran. 👋

Plan?

Why is async...

...code hard to grasp?

📸

This is why C# async code is hard...

Some... (raged but on point) community feedback on C# async design

(very boring) Important

Terminology Recap

System.Threading.Thread

System.Threading.Thread

System.Threading.ThreadPool

System.Threading.ThreadPool

System.Threading.Tasks.Task

System.Threading.Tasks.Task

System.Threading.SynchronizationContext

System.Threading.SynchronizationContext 1/2​

System.Threading.SynchronizationContext 2/2​

TaskScheduler

of System.Threading.Tasks

TaskScheduler

System.Threading.Tasks

How is a task scheduled?

Deadlock

Deadlock

(thread) Blocking

some e-z win tips...

PROTIP 1:

PROTIP 2:

PROTIP 3:

PROTIP 4:

Sync

VS Async VS

Parallel

Sync

VS Async VS

Parallel

That's it for the boooooring part... 🔨

demo: https://github.com/vmandic/you-still-dont-know-async-await

If this was too little... :-)

Takeaways

The end! 📸 Questions?

Async await is not a playground

More from Vedran Mandić

C# async ⚡️ await
is not your playground

This is why
C# async code is hard...

Some...
(raged but on point) community feedback on C# async design

System.Threading.SynchronizationContext 1/2

System.Threading.SynchronizationContext 2/2

demo:
https://github.com/vmandic/you-still-dont-know-async-await