How to retrieve the Exception of a ValueTask?

Question

The design of the upcoming C# 8 IAsyncEnumerable uses ValueTask and ValueTask<T> to communicate the potentially synchronous results back to the consumer logic. They both have the IsFaulted property, but unlike Task, there is no Exception property.

Is it even possible to have a ValueTask that doesn't hold a normal Task and is in the faulted or canceled state?

The ValueTask<T>.Result's documentation indicates calling it on a failed task will rethrow the contained Exception. Would the following code make thus sense to extract the Exception?

IAsyncEnumerable<int> asyncSequence = ...

ValueTask<bool> valueTask = asyncSequence.MoveNextAsync();

if (valueTask.IsFaulted) {
    // this has to work in a non-async method
    // there is no reason to block here or use the
    // async keyword
    try {
        var x = valueTask.Result;
    } catch (Exception ex) {
        // work with the exception
    }
}

ValueTask endTask = asyncSequence.DisposeAsync();

if (endTask.IsFaulted) {
    // there is no ValueTask.Result property
    // so the appoach above can't work
}

The non-generic ValueTask does not have a Result property. How can I extract the Exception then?

Generally, I suppose applying AsTask could be used for both extractions, however, I'd think it incurs allocation making the use of ValueTask questionable in the first place as I understand it.

Answer 1

The common way to block until completion of a Task or Task<T> is by invoking the Task.Wait() method.

If an exception is thrown, it will be a System.AggregatedException. The InnerExceptions property will have the collection of exceptions that caused the current exception.

Starting with .NET Framework 4.5 and .NET Core 1.0 there's also the GetAwaiter() method where you can invoke GetResult() on it's return value and will throw the first exception in the collection.

But you should never do that!!!

You should do this:

// this has to work in a non-async method
try {
    var x = await valueTask;
} catch (Exception ex) {
    // work with the exception
}

ValueTask and ValueTask<T> are used to avoid heap allocations when, potentially, the result is already available. It doesn't mean that it's safe to block on that task.

Answer 2

The response by the corefx team indicates this is indeed not possible directly and can't be easily added due to the need to change a companion interface and that would bring complications along.

The workaround suggested, as I guessed originally, to use AsTask to get the Exception.

IAsyncEnumerable<int> asyncSequence = ...

ValueTask<bool> valueTask = asyncSequence.MoveNextAsync();

if (valueTask.IsFaulted) {
    var ex = valueTask.AsTask().Exception;
    // work with the exception
}

ValueTask endTask = asyncSequence.DisposeAsync();

if (endTask.IsFaulted) {
    var ex = valueTask.AsTask().Exception;
    // work with the exception
}

Answer 3

Your best bet would be to use a try statement and attempt to catch the exception there and then handle it later... In regards to the ValueTask that doesn't hold a normal task, I would think that you'd be able to use lambda methods to resolve that.

try {
    var x = Task.Run(()=>{ ... });
}
catch (Exception ex) {
    // Do something with exception.
}

Or whatever the case would be for ValueTask. Considering C# has focused on Async tasks a lot these past builds, it would be safe to say that you would use lambda and event calls.

private void foo(ValueTask task) {
        lock (threadObj) {
            var x = Task.Run(()=> { return task.doWork(); }
            if (x == true) {
                // then do other work...
            }
            else {
                // handle 'exception'...
            }
        }
}