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C#•2y ago

❔ This is not a .NET bug, right? (async void)

Hi! Is this by design?

using System;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        DoStuff();
    }

    private static async void DoStuff()
    {
        try
        {
            await DoAsyncStuff();
        }
        catch
        {
            Console.WriteLine("catch");
        }
        finally
        {
            Console.WriteLine("finally");
        }
    }

    private static async Task DoAsyncStuff()
    {
        await Task.Delay(1);
        throw new Exception();
    }
}

using System;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        DoStuff();
    }

    private static async void DoStuff()
    {
        try
        {
            await DoAsyncStuff();
        }
        catch
        {
            Console.WriteLine("catch");
        }
        finally
        {
            Console.WriteLine("finally");
        }
    }

    private static async Task DoAsyncStuff()
    {
        await Task.Delay(1);
        throw new Exception();
    }
}

Neither catch nor finally is getting hit. I know that exceptions which are being raised in async void methods are being swallowed but didn't think that those in a try block would get swallowed aswell. For reference: https://stackoverflow.com/a/5383408 https://stackoverflow.com/a/16158374 ^ Those answers state that the exceptions are getting catched

50 Replies

ero•2y ago

you're not awaiting DoStuff()

qqdev•2y ago

Yeah, because you can't

FusedQyou•2y ago

Don't use async void That's your fix

qqdev•2y ago

async void is "fine" for event handlers

FusedQyou•2y ago

No it's not Don't use async void ever

qqdev•2y ago

ero•2y ago

i mean it's "fine" in the sense that there is literally no other option

FusedQyou•2y ago

https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md#async-void

GitHub

AspNetCoreDiagnosticScenarios/AsyncGuidance.md at master · davidfow...

This repository has examples of broken patterns in ASP.NET Core applications - AspNetCoreDiagnosticScenarios/AsyncGuidance.md at master · davidfowl/AspNetCoreDiagnosticScenarios

ero•2y ago

like for winforms for example you have to use async void for those events

FusedQyou•2y ago

Weird how there is an exception for a feature that is so error-prone But this does not look like an event handler

qqdev•2y ago

What is the alternative here? I can't change the signature btw My question is rather if this is a .NET bug or not

ero•2y ago

it's not

qqdev•2y ago

Cause the SO posts are stating that it works for them

ero•2y ago

the flow leaves Main before anything in it is executed so the program terminates

qqdev•2y ago

So the SO posts are wrong?

FusedQyou•2y ago

I mean, SO posts are wrong all the time Why can't you change the signature into a Task?

qqdev•2y ago

Makes sense, was just wondering! ty!

MODiX•2y ago

Ero#1111

REPL Result: Success

DoStuff();

static async void DoStuff()
{
try
{
    await DoAsyncStuff();
}
catch
{
    Console.WriteLine("catch");
}
finally
{
    Console.WriteLine("finally");
}
}

static async Task DoAsyncStuff()
{
await Task.Delay(1);
throw new Exception();
}

DoStuff();

static async void DoStuff()
{
try
{
    await DoAsyncStuff();
}
catch
{
    Console.WriteLine("catch");
}
finally
{
    Console.WriteLine("finally");
}
}

static async Task DoAsyncStuff()
{
await Task.Delay(1);
throw new Exception();
}

Compile: 613.317ms | Execution: 67.114ms | React with ❌ to remove this embed.

MODiX•2y ago

Ero#1111

REPL Result: Success

DoStuff();
await Task.Delay(1000);

static async void DoStuff()
{
try
{
    await DoAsyncStuff();
}
catch
{
    Console.WriteLine("catch");
}
finally
{
    Console.WriteLine("finally");
}
}

static async Task DoAsyncStuff()
{
await Task.Delay(1);
throw new Exception();
}

DoStuff();
await Task.Delay(1000);

static async void DoStuff()
{
try
{
    await DoAsyncStuff();
}
catch
{
    Console.WriteLine("catch");
}
finally
{
    Console.WriteLine("finally");
}
}

static async Task DoAsyncStuff()
{
await Task.Delay(1);
throw new Exception();
}

Console Output

catch
finally

catch
finally

Compile: 559.535ms | Execution: 1107.793ms | React with ❌ to remove this embed.

qqdev•2y ago

It is not my signature 3rd party code

FusedQyou•2y ago

Clone the repo and fix their mistake 😄

ero•2y ago

that's a breaking change

qqdev•2y ago

Easier said than done ;D (in a real world)

FusedQyou•2y ago

I didn't feel like typing out "Clone their repo and create a second proper asynchronous method and make the current one obsolete"

qqdev•2y ago

So tl;dr is: Exceptions in async void methods are getting swallowed no matter what

Anchy•2y ago

makes sense to why the code works fine for me on my piece of shit laptop, as I'm getting an output

FusedQyou•2y ago

That's why it's bad

qqdev•2y ago

No doubt

FusedQyou•2y ago

When in doubt about asynchronous practices, consult this: https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md

GitHub

AspNetCoreDiagnosticScenarios/AsyncGuidance.md at master · davidfow...

This repository has examples of broken patterns in ASP.NET Core applications - AspNetCoreDiagnosticScenarios/AsyncGuidance.md at master · davidfowl/AspNetCoreDiagnosticScenarios

FusedQyou•2y ago

Very interesting explanation on many things

qqdev•2y ago

It does not cover event handlers tho, right?

ero•2y ago

don't use events

qqdev•2y ago

;D Wait a second. The exception is getting catched if you give it enough time (added a delay to the Main()). Which makes sense The question is why it is not catched in our app then. It is long-running and not immediately exiting

FusedQyou•2y ago

I mean, adding a delay to the main is basically the same as awaiting the task if it had one, without actually handling any exception I would really just update the library you're trying to use What is it anyway?

qqdev•2y ago

Okay, what could be the scenario here then? <:PB_peepo_think:769654147041067028> App is not exiting immediately, but exception is not getting catched That's prob why the SO posts seem to be somewhat correct (because it works under circumstances)

FusedQyou•2y ago

I mean, I don't think you will find a fix for this You can add a boolean that your async void method handles and your main method will block until it is changed? Can I see the library pls

qqdev•2y ago

An explanation would be more than enough lul

lul

Current fix idea is to use ContinueWith(...) But we are caring about the reason because that has an influence on the fix

FusedQyou•2y ago

What ContinueWith() requires a Task Which you don't have

qqdev•2y ago

I have one in the async void method Which is why the method is marked with async in the first place

FusedQyou•2y ago

I thought you could not change the method Or the signature

qqdev•2y ago

One can always add async in this case Don't have to touch the 3rd party code

FusedQyou•2y ago

Oh, like that You have an interface to implement So the whole problem is that you have an interface to abide to, but you have async behaviour

qqdev•2y ago

public async void MyHandler(...)
{
    await DoAsyncStuff(...).ContinueWith(task => ...);
}

public async void MyHandler(...)
{
    await DoAsyncStuff(...).ContinueWith(task => ...);
}

FusedQyou•2y ago

You should just keep the method void, not async void, and then store the Task in memory Or, you know, block the whole thing But don't fix it with async void lol

qqdev•2y ago

Wdym by store the task in memory?

FusedQyou•2y ago

You store the Task retrieved from DoAsyncStuff in memory Or discard it and use ContinueWith to get the result of the task and catch errors There is literally no point in using await here if the whole method is async void

qqdev•2y ago

Oh yeah, for sure Got ya

FusedQyou•2y ago

Still, the fact that the library does not allow an asynchronous alternative is just bad

qqdev•2y ago

EventHandler's in a nutshell I guess? ;D

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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