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Async Enumerables with Cancellation

 

In this post, I’ll explain how to produce and consume async enumerables with support for cancellation. Since originally publishing this post, we’ve added support in the language for a new attribute which solve this problem more elegantly. I’ve added a section detailing the new method.

Some context

Visual Studio 2019 (currently in preview) includes a preview of C# 8.0 and the async-streams feature.

Three parts compose this feature:

  1. async-iterator methods: you can write methods with the async modifier, returning either IAsyncEnumerable or IAsyncEnumerator, and using both yield and await syntax.
  2. await foreach: you can asynchronously enumerate collections that implement IAsyncEnumerable (or implement equivalent APIs).
  3. await using: you can asynchronously dispose resources that implement IAsyncDisposable.

await foreach follows a similar execution pattern as its synchronous sibling foreach: it first gets an enumerator from the enumerable (by calling GetAsyncEnumerator(), then repeatedly does await MoveNextAsync() on the enumerator and gets the item with Current until the enumerator is exhausted.

Here’s the code generated for an await foreach:

E e = ((C)(x)).GetAsyncEnumerator();
try
{
    while (await e.MoveNextAsync())
    {
        V v = (V)(T)e.Current;
        // body
    }
}
finally
{
    await e.DisposeAsync();
}

You may notice in the relevant APIs (copied below) that GetAsyncEnumerator accepts a CancellationToken parameter. But await foreach doesn’t make use of this parameter (it passes a default value).

This raises two questions: 1) how do you write an async enumerable with support for cancellation? and 2) how do you consume one?

Writing an async enumerable supporting cancellation (original method)

Let’s say that you intend to write IAsyncEnumerable<int> GetItemsAsync(int maxItems) supporting cancellation.

You cannot just write an async iterator method async IAsyncEnumerable<int> GetItemsAsync(int maxItems) because that does not give you access to any cancellation token.

You also cannot write an async iterator method async IAsyncEnumerable<int> GetItemsAsync(int maxItems, CancellationToken token) because:

  1. if a method has its own cancellation token and wants to enumerate an async enumerable it received, it could not use the token it wants with that enumerable (the cancellation token would be already built into the enumerable),
  2. the same cancellation token would be used in every enumerator when the collection is enumerated multiple times,

So instead, you need to implement the enumerable yourself and put your business logic in async IAsyncEnumerator<int> GetAsyncEnumerable(CancellationToken cancellationToken).

Here’s what that looks like:

public static IAsyncEnumerable<int> GetItemsAsync(int maxItems)
    => new MyCancellableCollection(maxItems);
    
class MyCancellableCollection : IAsyncEnumerable<int>
{
    private int _maxItems;
    internal MyCancellableCollection(int maxItems)
        => _maxItems = maxItems;
        
    public async IAsyncEnumerator<int> GetAsyncEnumerator(CancellationToken cancellationToken)
    {
        // Your method body using:
        // - `_maxItems`
        // - `cancellationToken.ThrowIfCancelled();`
        // - `await` and `yield` constructs
    }
}

We recognize that this involves boilerplate. We are considering some language design options to further simplify this. Since originally publishing this, we’ve solved this problem more elegantly by extending the language. The next section explains the updated design.

Writing an async enumerable supporting cancellation (improved method)

In an updated preview of C# 8.0 (shipping in Visual Studio 2019 version 16.1), we’ll be adding support for the [EnumeratorCancellation] token. The attribute allows you to write an async-iterator method, returning IAsyncEnumerable<T> as you intend, but tells the compiler to store the token from GetAsyncEnumerator(CancellationToken) into one of your method’s parameters.

In the above example, you would just declare the method as async IAsyncEnumerable<int> GetItemsAsync(int maxItems, [EnumeratorCancellation] CancellationToken token). Because of the attribute, the token parameter will be set to a synthesized cancellation token that combines two token: the one passed as an argument to the method, and the other given to GetAsyncEnumerator. This synthesized token gets cancelled when either of the two given tokens is cancelled.

async IAsyncEnumerable<int> GetItemsAsync(int maxItems, [EnumeratorCancellation] CancellationToken token)
{
        // Your method body using:
        // - `_maxItems`
        // - `token.ThrowIfCancelled();`
        // - `await` and `yield` constructs
}

Note: in dev16.1 preview5, we have not yet implemented this method of combining tokens, we took a simpler approach whereby any non-default token given to GetAsyncEnumerator will override the token passed as an argument. I expect to implement the more elaborate method of combining tokens in preview6 timeframe.

Consuming an async enumerable with cancellation

With the above implementation, if you wrote await foreach (var item in GetItemsAsync(maxItems: 10)) ..., a default cancellation token would be passed to the cancellable method.

Users of enumerables could try and expand the low-level code for an await foreach to pass a token, but that’s a terrible solution (defeats the purpose of await foreach).

To help with this, we provide a WithCancellation<T>(this IAsyncEnumerable<T> source, CancellationToken cancellationToken) extension method. It allows you to pass your token in:

await foreach (var item in GetItemsAsync(maxItems: 10).WithCancellation(token)) ...

This helper method wraps the enumerable from GetItemsAsync along with the given cancellation token. When GetAsyncEnumerator() is invoked on this wrapper, it calls GetAsyncEnumerator(token) on the underlying enumerable.

Appendix: relevant interfaces

using System.Threading;

namespace System.Collections.Generic
{
    /// <summary>Exposes an enumerator that provides asynchronous iteration over values of a specified type.</summary>
    /// <typeparam name="T">The type of values to enumerate.</typeparam>
    public interface IAsyncEnumerable<out T>
    {
        /// <summary>Returns an enumerator that iterates asynchronously through the collection.</summary>
        /// <param name="cancellationToken">A <see cref="CancellationToken"/> that may be used to cancel the asynchronous iteration.</param>
        /// <returns>An enumerator that can be used to iterate asynchronously through the collection.</returns>
        IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default);
    }

    /// <summary>Supports a simple asynchronous iteration over a generic collection.</summary>
    /// <typeparam name="T">The type of objects to enumerate.</typeparam>
    public interface IAsyncEnumerator<out T> : IAsyncDisposable
    {
        /// <summary>Advances the enumerator asynchronously to the next element of the collection.</summary>
        /// <returns>
        /// A <see cref="ValueTask{Boolean}"/> that will complete with a result of <c>true</c> if the enumerator
        /// was successfully advanced to the next element, or <c>false</c> if the enumerator has passed the end
        /// of the collection.
        /// </returns>
        ValueTask<bool> MoveNextAsync();

        /// <summary>Gets the element in the collection at the current position of the enumerator.</summary>
        T Current { get; }
    }
    
    /// <summary>Provides a mechanism for releasing unmanaged resources asynchronously.</summary>
    public interface IAsyncDisposable
    {
        /// <summary>
        /// Performs application-defined tasks associated with freeing, releasing, or
        /// resetting unmanaged resources asynchronously.
        /// </summary>
        ValueTask DisposeAsync();
    }
}

Original code for IAsyncEnumerable, IAsyncEnumerator and IAsyncDisposable.

For further details, see the async-streams design doc.

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