AsyncEnumerable in C#: The importance of EnumeratorCancellation attribute

Modern applications often need to process large datasets or streams of data asynchronously. When we need to iterate through such data without loading everything at once, we've traditionally used IEnumerable. But what if our data access is inherently asynchronous? Enter IAsyncEnumerable<T>, introduced in C# 8.0 and .NET Core 3.0, designed specifically for asynchronous streaming scenarios.

In this post, we'll explore IAsyncEnumerable<T> and why the EnumeratorCancellation attribute with a CancellationToken is crucial for writing robust, cancellable asynchronous code.

What is IAsyncEnumerable<T>?

IAsyncEnumerable<T> is an interface that represents a sequence of elements that can be asynchronously enumerated. It's the asynchronous counterpart to the familiar IEnumerable<T> interface.

The beauty of IAsyncEnumerable<T> is that it allows you to:

• Perform asynchronous operations while iterating through a sequence
• Yield results as they become available
• Handle cancellation gracefully

The need for cancellation

Why do we need cancellation support in asynchronous operations? Consider these scenarios:

1. A user navigates away from a web page that's fetching data
2. A search operation is taking too long, and the user cancels it
3. A service is shutting down while operations are in progress

Without proper cancellation support, resources can be wasted, and applications may become unresponsive or leak resources.

The EnumeratorCancellation attribute

The EnumeratorCancellation attribute is where the magic happens. When applied to a CancellationToken parameter in an async iterator method, it tells the compiler to pass this token to the GetAsyncEnumerator method when the iterator is being consumed with await foreach.

Let's look at a simple example:

Why is the EnumeratorCancellation attribute important?

Without the EnumeratorCancellation attribute, there's a disconnect between the cancellation token passed to the async iterator method and the one passed to GetAsyncEnumerator when consuming the enumerable. The EnumeratorCancellation attribute ensures that this token flows into your async iterator method, allowing you to react to cancellation requests from the consumer.

To help you avoid this mistake, the compiler issues a  CS8425 warning as a helpful reminder to properly implement cancellation in your async iterator methods.

The warning message typically looks like this:

warning CS8425: Async-iterator method 'MyMethod' has parameters that are declared with 'CancellationToken' but are not decorated with the 'EnumeratorCancellation' attribute, and will not be passed the cancellation token from 'WithCancellation' method calls

This warning occurs when you define an async iterator method that returns IAsyncEnumerable<T> and includes a CancellationToken parameter, but you haven't decorated that parameter with the [EnumeratorCancellation] attribute.

Consider this code that would trigger CS8425:

When you use this method with WithCancellation:

The cancellation token from WithCancellation won't be passed to your method's token parameter, making cancellation ineffective.

The solution is straightforward—add the [EnumeratorCancellation] attribute to your CancellationToken parameter:

Ignoring this warning can lead to subtle bugs where cancellation appears to be implemented but doesn't actually work. Your code will compile and run, but cancellation requests from consumers will be ignored, potentially leading to:

• Resource leaks
• Unresponsive applications
• Operations that continue running long after the user has requested cancellation

Conclusion

IAsyncEnumerable<T> bridges an important gap in .NET's asynchronous programming model. The EnumeratorCancellation attribute is a critical component that ensures cancellation requests flow correctly through your code, allowing for responsive, resource-efficient applications.

When working with asynchronous streams, always remember to:

• Use the EnumeratorCancellation attribute for your cancellation token parameters
• Check for cancellation regularly
• Propagate the token to inner operations
• Pay attention to compiler warning CS8425 as it indicates a potential cancellation issue

By following these practices, you'll create asynchronous streaming code that's both powerful and robust.