I have Producer, Operator, and Consumer.

Producer generates IAsyncEnumerable<TItem>. As example:

interface IProducer<TItem> : IAsyncDisposable
{
    IAsyncEnumerable<TItem> GetItemsAsync();
}

The Operator receives a request from the Consumer, creates an instance of the Producer, and streams data from it to the Consumer. The Operator acts as an intermediary between them.

interface IOperator<TItem> : IProducer<TItem>, IAsyncDisposable;

If Producer fails(into exception), Operator should:

1. Catch the exception.
2. Dispose of the Producer.
3. Re-throw the exception to Consumer.

The main issue is handling exceptions inside the Operator and propagating them to the Consumer. What’s the best way to handle this? Since yield return cannot be used inside a try block with a catch clause, how can this be properly implemented?

I have Producer, Operator, and Consumer.

Producer generates IAsyncEnumerable<TItem>. As example:

interface IProducer<TItem> : IAsyncDisposable
{
    IAsyncEnumerable<TItem> GetItemsAsync();
}

The Operator receives a request from the Consumer, creates an instance of the Producer, and streams data from it to the Consumer. The Operator acts as an intermediary between them.

interface IOperator<TItem> : IProducer<TItem>, IAsyncDisposable;

If Producer fails(into exception), Operator should:

1. Catch the exception.
2. Dispose of the Producer.
3. Re-throw the exception to Consumer.

The main issue is handling exceptions inside the Operator and propagating them to the Consumer. What’s the best way to handle this? Since yield return cannot be used inside a try block with a catch clause, how can this be properly implemented?

• c#
• exception
• iasyncenumerable

• As a side note, it's not clear what is the advantage of adding the Operator in the mix. Whoever has the responsibility to dispose the Operator, it could instead dispose the Producer directly. – Theodor Zoulias Commented 21 hours ago
• Can we see a concrete example of an IOperator? Why do you need a catch block, why not just a finally for the dispose? Or better just have a using for the IProducer – Charlieface Commented 20 hours ago

1 Answer 1

All you have to do is to dispose the Producer in a finally block. Using the yield in a try-finally block is allowed.

try
{
    await foreach (Item item in _producer.GetItemsAsync())
    {
        yield return item;
    }
}
finally
{
    await _producer.DisposeAsync();
}

In this case the finally block runs as follows:

1. In case the inner await foreach loop completes successfully, meaning that neither the producer failed, nor the consumer abandoned the enumeration, then the finally runs during the last MoveNextAsync(), which returns false.
2. In case the producer fails, the finally runs during the last MoveNextAsync(), before the caught exception is rethrown.
3. In case the consumer abandons the enumeration before its natural completion, for example intentionally with a break; or unintentionally by an exception thrown inside the consuming await foreach loop, the finally runs when the consumer disposes the enumerator of the generated IAsyncEnumerable<TItem>. The await foreach statement creates automatically an enumerator, and makes sure that it is always disposed by calling await enumerator.DisposeAsync() in a finally block.

Using a try-finally block as shown above is useful if you want to have finer control over the conditions that mandate the disposal of the producer. In the common case where you want to dispose it unconditionally, you can use instead the simpler await using statement:

await using (_producer)
{
    await foreach (Item item in _producer.GetItemsAsync())
    {
        yield return item;
    }
}