Shooting yourself in the foot with C# Tasks - ContinueWith

Accidentally swallowing exceptions with C# async ContinueWith() is a real possibility.

I’ve accidentally done the same recently on a project.

The idea was that once a method finished running, I wanted to execute a log of the task that was executed.

So my Controller Action looked something like this:

1
2
3
4
5
6
7
[HttpDelete]
public async Task<HttpResponseMessage> DeleteSomething(int id)
{
    await _repository.DeleteSomething(id)
        .ContinueWith(task => _log.Log(task));
    return Request.CreateResponse(HttpStatusCode.OK);
}

The delete would go and run the query against the database and delete some records in Azure Table Storage.

The Log method would just read the object resulting from the task completing and finish.

What would happen when DeleteSomething throws an exception? ContinueWith would get passed the faulted Task and if you didn’t retrow at that point, it would go on and return an HTTP 200.

Wow. That’s bad. It’s like a highly sophisticated On Error Resume Next. Welcome to 1995.

Let’s fix this. I’m expecting to run this Task only when it succeed. So let’s make sure I use the right overload.

1
2
3
4
5
6
7
[HttpDelete]
public async Task<HttpResponseMessage> DeleteSomething(int id)
{
    await _repository.DeleteSomething(id)
        .ContinueWith(task => _log.Log(task), TaskContinuationOptions.OnlyOnRanToCompletion);
    return Request.CreateResponse(HttpStatusCode.OK);
}

If I keep doing stupid stuff like this, I might turn this into a series of what NOT to do.

If you encountered something similar, please let me know in the comments!