使用 Task.ContinueWith 时如何避免嵌套的 AggregateException？

Question

I have an async method in a .NET 4.5 C# component:

public async Task<T> GetResultAsync()
{
    return PerformOperationAsync();
}

If PerformOperationAsync throws an exception, then I can catch an AggregateException on a client side, unwrap it and get the original exception thrown.

However, if I have slightly more complicated code:

public async Task<T> GetResultAsync()
{
    return PerformOperationAsync().ContinueWith(x =>
    {
        var result = x.Result;
        return DoSomethingWithResult(result);
    }, cancellationToken);
}

... then in case an exception occurs, the client catches a nested AggregateException, so it has to flatten it prior getting the original one.

Should this behavior be avoided or does client have to expect possibly nested AggregateException and call Flatten to unwrap all its levels? And if this behavior should be avoided by the component developer, then what's the right way to deal with it in ContinueWith scenario? I have plenty of similar situations, so I am trying to find the most lightweight method of handling them.

Answer 1

C#5 async/await will help you deal with continuations and proper exception handling while simplifying the code.

public async Task<T> GetResultAsync()
{
    var result = await PerformOperationAsync().ConfigureAwait(false);
    return DoSomethingWithResult(result);
}

Your method is already marked as async, is it intended ?

---

To keep the continuation you can provide a TaskContinuationOptions with OnlyOnRanToCompletion value :

PerformOperationAsync().ContinueWith(x =>
{
    var result = x.Result;
    return DoSomethingWithResult(result);
}, TaskContinuationOptions.OnlyOnRanToCompletion);

or use the awaiter to raise the original exception

PerformOperationAsync().ContinueWith(x =>
{
    var result = x.GetAwaiter().GetResult();
    return DoSomethingWithResult(result);
}, cancellationToken);

Answer 2

In addition to Guillaume's answer, I am using a little extension method

public static class Ext
{
    /// <summary>
    /// Wait for task synchronously, then return result. Avoid AggregateExceptions 
    /// as it would be generated by asyncTask.Result.
    /// </summary>
    public static T SyncResult<T>(this Task<T> asyncTask) 
                                    => asyncTask.GetAwaiter().GetResult();
}

Instead of var result = x.Result; , use it like this:

var result = x.SyncResult();

It does the same as Guillaume explained, but is shorter to use and easier to memorize.