Elegantly handle task cancellation

Question

When using tasks for large/long running workloads that I need to be able to cancel I often use a template similar to this for the action the task executes:

public void DoWork(CancellationToken cancelToken)
{
    try
    {
        //do work
        cancelToken.ThrowIfCancellationRequested();
        //more work
    }
    catch (OperationCanceledException)
    {
        throw;
    }
    catch (Exception ex)
    {
        Log.Exception(ex);
        throw;
    }
}

The OperationCanceledException should not be logged as an error but must not be swallowed if the task is to transition into the cancelled state. Any other exceptions do not need to be dealt with beyond the scope of this method.

This always felt a bit clunky, and visual studio by default will break on the throw for OperationCanceledException (though I have 'break on User-unhandled' turned off now for OperationCanceledException because of my use of this pattern).

UPDATE: It's 2021 and C#9 gives me the syntax I always wanted:

public void DoWork(CancellationToken cancelToken)
{
    try
    {
        //do work
        cancelToken.ThrowIfCancellationRequested();
        //more work
    }
    catch (Exception ex) when (ex is not OperationCanceledException)
    {
        Log.Exception(ex);
        throw;
    }
}

Ideally I think I'd like to be able to do something like this:

public void DoWork(CancellationToken cancelToken)
{
    try
    {
        //do work
        cancelToken.ThrowIfCancellationRequested();
        //more work
    }
    catch (Exception ex) exclude (OperationCanceledException)
    {
        Log.Exception(ex);
        throw;
    }
}

i.e. have some sort of exclusion list applied to the catch but without language support that is not currently possible (@eric-lippert: c# vNext feature :)).

Another way would be through a continuation:

public void StartWork()
{
    Task.Factory.StartNew(() => DoWork(cancellationSource.Token), cancellationSource.Token)
        .ContinueWith(t => Log.Exception(t.Exception.InnerException), TaskContinuationOptions.OnlyOnFaulted | TaskContinuationOptions.ExecuteSynchronously);
}

public void DoWork(CancellationToken cancelToken)
{
    //do work
    cancelToken.ThrowIfCancellationRequested();
    //more work
}

but I don't really like that as the exception technically could have more than a single inner exception and you don't have as much context while logging the exception as you would in the first example (if I was doing more than just logging it).

I understand this is a bit of a question of style, but wondering if anyone has any better suggestions?

Do I just have to stick with example 1?

Solution 1

So, what's the problem? Just throw away catch (OperationCanceledException) block, and set proper continuations:

var cts = new CancellationTokenSource();
var task = Task.Factory.StartNew(() =>
    {
        var i = 0;
        try
        {
            while (true)
            {
                Thread.Sleep(1000);

                cts.Token.ThrowIfCancellationRequested();

                i++;

                if (i > 5)
                    throw new InvalidOperationException();
            }
        }
        catch
        {
            Console.WriteLine("i = {0}", i);
            throw;
        }
    }, cts.Token);

task.ContinueWith(t => 
        Console.WriteLine("{0} with {1}: {2}", 
            t.Status, 
            t.Exception.InnerExceptions[0].GetType(), 
            t.Exception.InnerExceptions[0].Message
        ), 
        TaskContinuationOptions.OnlyOnFaulted);

task.ContinueWith(t => 
        Console.WriteLine(t.Status), 
        TaskContinuationOptions.OnlyOnCanceled);

Console.ReadLine();

cts.Cancel();

Console.ReadLine();

TPL distinguishes cancellation and fault. Hence, cancellation (i.e. throwing OperationCancelledException within task body) is not a fault.

The main point: do not handle exceptions within task body without re-throwing them.

Solution 2

Here is how you elegantly handle Task cancellation:

Handling "fire-and-forget" Tasks

var cts = new CancellationTokenSource( 5000 );  // auto-cancel in 5 sec.
Task.Run( () => {
    cts.Token.ThrowIfCancellationRequested();

    // do background work

    cts.Token.ThrowIfCancellationRequested();

    // more work

}, cts.Token ).ContinueWith( task => {
    if ( !task.IsCanceled && task.IsFaulted )   // suppress cancel exception
        Logger.Log( task.Exception );           // log others
} );

Handling await Task completion / cancellation

var cts = new CancellationTokenSource( 5000 ); // auto-cancel in 5 sec.
var taskToCancel = Task.Delay( 10000, cts.Token );  

// do work

try { await taskToCancel; }           // await cancellation
catch ( OperationCanceledException ) {}    // suppress cancel exception, re-throw others

Solution 3

You could do something like this:

public void DoWork(CancellationToken cancelToken)
{
    try
    {
        //do work
        cancelToken.ThrowIfCancellationRequested();
        //more work
    }
    catch (OperationCanceledException) when (cancelToken.IsCancellationRequested)
    {
        throw;
    }
    catch (Exception ex)
    {
        Log.Exception(ex);
        throw;
    }
}

Solution 4

C# 6.0 has a solution for this..Filtering exception

int denom;

try
{
     denom = 0;
    int x = 5 / denom;
}

// Catch /0 on all days but Saturday

catch (DivideByZeroException xx) when (DateTime.Now.DayOfWeek != DayOfWeek.Saturday)
{
     Console.WriteLine(xx);
}

View more solutions

Comments

[Dennis]

This: catch (OperationCanceledException) {} will set the task's status as RanToCompletion, not as Canceled. There are use cases, when this is a significant difference.

[Eamon]

My only issue with the continuation method was that I loose context. e.g. If I was processing a list of Items and I needed to log how far through the collection I was when the exception was thrown. I did miss something in my original question which I have fied now and that was a rethrow of the Exception ex to allow the task to transition to the Faulted state.

[Casey Anderson]

There are some simpler answers below.

[Spi]

The keyword is actually 'when' not 'if'. The syntax (for the OP) would be: catch( Exception ex ) when (!(ex is OperationCanceledException))

[Ohad Schneider]

I prefer a more strict check: catch (OperationCanceledException e) when (e.CancellationToken == cancelToken)

[alv]

With C# 9.0 a nicer syntax: catch (Exception ex) when (ex is not OperationCanceledException)

[Theodor Zoulias]

@OhadSchneider in general (not in the specific trivial example) there is no guarantee that the cancelToken will be propagated through the OperationCanceledException.CancellationToken. Linked tokens are usually created internally by asynchronous methods, hiding the identity of the originating CancellationToken. Checking the condition when (cancelToken.IsCancellationRequested) is the best you can do in most cases IMHO.