Asynchronous programming Deadlock All The Things!.

Asynchronous programming

Deadlock All The Things!

/ Copyright ©2014 by Readify Pty Ltd2Page

Filip [email protected]

http://fekberg.com

+61 (401) 157-608

@fekberg

C# Smorgasbord

mailto:[email protected]

http://blog.namesurname.com/

Page / Copyright ©2014 by Readify Pty Ltd3

Asynchronous programming?› Processing allowed before current execution is done, such as read/write to disk

› So is this the same as parallel? Not quite.

@fekberg


Async + Parallel12:38

Load app content

Sort data

Process

records

Asynchronous task

@fekberg


Async? Never heard of it.› Ajax - Asynchronous JavaScript and XML› Request data› Handle the response when available

@fekberg


Introducing Async and Await› Two new Contextual Keywords as of .NET 4.5

› Meaning it does all the hard work for you

public async Task RunAsync(){ await Task.Run(() => { });}

@fekberg


Marking a method as Async› This will indicate that the method will be executed from within a state machine

› Adds overhead

› If it is not actually awaiting something, don’t mark it as async!

› Simply marking as async doesn’t make it async!

@fekberg


What do you think happens?private void Button_Click(){ try { Run(); } catch (Exception ex) { }}

public void Run(){ throw new Exception();}

@fekberg


What do you think happens?private void Button_Click(){ try { RunAsync(); } catch (Exception ex) { }}

public async void RunAsync(){ throw new Exception();}

@fekberg


The second one crashes!› Async void is EVIL!

› Marking a method as async wraps it inside a state machine[AsyncStateMachine(typeof(<RunAsync2>d__0)), DebuggerStepThrough]public void RunAsync2(){ <RunAsync2>d__0 d__; d__.<>4__this = this; d__.<>t__builder = AsyncVoidMethodBuilder.Create(); d__.<>1__state = -1; d__.<>t__builder.Start<<RunAsync2>d__0>(ref d__);}

@fekberg


How do we handle this?› Use Task as a return type instead

› Always await your async callsprivate async void Button_Click(){ try { await RunAsync(); } catch (Exception ex) { }}

private void Button_Click(){ RunAsync();}

public async Task RunAsync(){ throw new Exception();}

Silently fails

“Validates” and lets you catch any possible exceptions

@fekberg


Proper Async methodpublic async Task BoilEggsAsync(int amountOfEggs){ await BoilAsync(amountOfEggs); }

public Task BoilEggsAsync(int amountOfEggs){ <BoilEggsAsync>d__5 d__; d__.<>4__this = this; d__.amountOfEggs = amountOfEggs; d__.<>t__builder = AsyncTaskMethodBuilder.Create(); d__.<>1__state = -1; d__.<>t__builder.Start<<BoilEggsAsync>d__5>(ref d__); return d__.<>t__builder.Task;}


Awaiting a result› Awaiting a task will schedule a continuation block

› Everything after the awaiting task will be executed when the task is done executingpublic async Task RunAsync(){ var resultTask = Task<string>.Run(() => { Thread.Sleep(2000);

return "Hello World!"; });

var result = await resultTask;}

Back on the UI thread after the waiting is done

@fekberg


Example of deadlocking› Wait() for an asynchronous task on the UI thread, making it impossible for the state machine to signal that it is completed!public async Task RunAsync()…

RunAsync().Wait();

@fekberg


Things to consider› Don’t mark void methods as asynchronous, you will have

no way of tracking it

› Tasks swallow Exceptions!

› Don’t explicitly .Wait() this is an easy path to deadlocking

› Follow the naming convention MyMethodAsync

› Don’t lie by wrapping synchronous/blocking code in async methods

› The continuation is on the calling thread!

› Async lambdas: @fekberg

async () => { await Task.Delay(2000); }


Summary› Async is powerful

› Using Async wrong may deadlock your application

› Can be hard to debug

› Await all your tasks to verify the execution!

@fekberg


Questions?


Thank [email protected]

http://fekberg.com

+61 (401) 157-608

@fekberg

C# Smorgasbord

mailto:[email protected]

http://blog.namesurname.com/

Asynchronous programming Deadlock All The Things!.

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