C#•3y ago

Task.Run swallows the exceptions [Answered]

Pretty self explanatory. The minimal reproducible example throws an exception in the Task.Run(...) and it doesn't rethrow it out of the Task.Run(...) scope.

public static class PollyRetry
{
    public static T Do<T>(Func<T> action, TimeSpan retryWait, int retryCount = 0)
    {
        var policyResult = Policy
            .Handle<Exception>()
            .WaitAndRetry(retryCount, retryAttempt => retryWait)
            .ExecuteAndCapture(action);

        if (policyResult.Outcome == OutcomeType.Failure)
        {
            throw policyResult.FinalException;
        }

        return policyResult.Result;
    }

    public static async Task<T> DoAsync<T>(Func<Task<T>> action, TimeSpan retryWait, int retryCount = 0)
    {
        var policyResult = await Policy
            .Handle<Exception>()
            .WaitAndRetryAsync(retryCount, retryAttempt => retryWait)
            .ExecuteAndCaptureAsync(action);

        if (policyResult.Outcome == OutcomeType.Failure)
        {
            throw policyResult.FinalException;
        }

        return policyResult.Result;
    }
}

public sealed class MinimalReproducibleCode
{
    public async Task StartWithRetry()
    {
        await Retry.DoAsync(() => Task.FromResult(StartAsync()), TimeSpan.FromSeconds(5), 5);
    }
    
    public Task StartAsync()
    {
        Console.WriteLine("This has just started");
        
        _ = Task.Run(() =>
        {
            while (true)
            {
                Console.WriteLine("Code is working");

                throw new DivideByZeroException();
            }
        });
        
        return Task.CompletedTask;
    }
}

public static class PollyRetry
{
    public static T Do<T>(Func<T> action, TimeSpan retryWait, int retryCount = 0)
    {
        var policyResult = Policy
            .Handle<Exception>()
            .WaitAndRetry(retryCount, retryAttempt => retryWait)
            .ExecuteAndCapture(action);

        if (policyResult.Outcome == OutcomeType.Failure)
        {
            throw policyResult.FinalException;
        }

        return policyResult.Result;
    }

    public static async Task<T> DoAsync<T>(Func<Task<T>> action, TimeSpan retryWait, int retryCount = 0)
    {
        var policyResult = await Policy
            .Handle<Exception>()
            .WaitAndRetryAsync(retryCount, retryAttempt => retryWait)
            .ExecuteAndCaptureAsync(action);

        if (policyResult.Outcome == OutcomeType.Failure)
        {
            throw policyResult.FinalException;
        }

        return policyResult.Result;
    }
}

public sealed class MinimalReproducibleCode
{
    public async Task StartWithRetry()
    {
        await Retry.DoAsync(() => Task.FromResult(StartAsync()), TimeSpan.FromSeconds(5), 5);
    }
    
    public Task StartAsync()
    {
        Console.WriteLine("This has just started");
        
        _ = Task.Run(() =>
        {
            while (true)
            {
                Console.WriteLine("Code is working");

                throw new DivideByZeroException();
            }
        });
        
        return Task.CompletedTask;
    }
}

12 Replies

Scratch•3y ago

uh yeah, that code is a little sus you're firing and forgetting the task. You need to await it for the exception to be thrown

HulkstanceOP•3y ago

yeah, but isn't there another way? since in my real use case I cannot remove the Task.Run

Scratch•3y ago

No. The exception is happening outside the call stack what are you trying to do at a high level? I know Polly can be used for retrying operations and such

HulkstanceOP•3y ago

this is my real use case scenario

message.txt

HulkstanceOP•3y ago

it shouldn't block the SendAsync is currently one statement but it should be a Task.Run loop too I was thinking of

var sending = Task.Run(...);
var processing = Task.Run(...);
var receiving = Task.Run(...);

await await Task.WhenAny(sending, processing, receiving);

var sending = Task.Run(...);
var processing = Task.Run(...);
var receiving = Task.Run(...);

await await Task.WhenAny(sending, processing, receiving);

and wrapping the StartAsync logic in a Task.Run(...);

Scratch•3y ago

here we go, wew. Those Task.Runs should instead be Polly policies or whatever they're called. You need to break it down into what specifically needs to be retried. oh I see you're using Task.Run because you want it to run stuff in the background?

HulkstanceOP•3y ago

Scratch•3y ago

put your polly retries inside of those tasks around whatever operation you need retried the body of the while loop maybe

HulkstanceOP•3y ago

yeah, but I need to recreate the whole ClientWebSocket instance at each reconnect wouldn't that retry the loop only? ClientWebSocket has to be reinstantiated once it throws, stops or whatever

Scratch•3y ago

yeah. You'll essentially need a Task.Run for a watchdog. I guess it'll await those worker tasks (also I recommend passing around cancellation tokens, otherwise you can't stop the execution)

HulkstanceOP•3y ago

true, I didn't implement those yet https://github.com/teamhitori/mulplay-container-web/blob/eb39e75e74c44d3c290bcc30737f8b861031fb37/dotnet/Components/WebSocketService.cs quick search on github gave me this result someone did quite the same

Accord•3y ago

✅ This post has been marked as answered!

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Task.Run swallows the exceptions [Answered]

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