Passing a cancellation token to an IAsyncEnumerable

I talked about the concept of asynchronous streams before. Thanks to the introduction of the IAsyncEnumerable interface, it allowed us to combine the richness of LINQ with the readibility of async-await to asynchronously access databases, microservices and remote APIs.

	[ApiController]
	[Route("[controller]")]
	public class ProductsController : ControllerBase
	{
	public async IAsyncEnumerable<Product> GetOnSaleProductsAsync()
	{
	var products = _repository.GetProductsAsync();
	await foreach (var product in products)
	{
	if (product.IsOnSale)
	{
	yield return product;
	}
	}
	}
	}

view raw ABetterProductsController.cs hosted with ❤ by GitHub

As enumerating async streams will typically result in remote asynchronous operations it can take a while to complete. I was wondering how we could cancel these async operations?

To support this use case, IAsyncEnumerable has an extension method ‘WithCancellation()’. The CancellationToken will be passed on to the  IAsyncEnumerable.GetEnumerator method.

	[ApiController]
	[Route("[controller]")]
	public class ProductsController : ControllerBase
	{
	public async IAsyncEnumerable<Product> GetOnSaleProductsAsync(CancellationToken cancellationToken)
	{
	var products = _repository.GetProductsAsync();
	await foreach (var product in products.WithCancellation(cancellationToken))
	{
	if (product.IsOnSale)
	{
	yield return product;
	}
	}
	}
	}

view raw Cancellation.cs hosted with ❤ by GitHub

More information about what’s happening behind the scenes can be found here.

Remark: There are some further improvements on the way in ASP.NET Core 6 regarding consuming IAsyncEnumerable streams as mentioned in this post: https://www.tpeczek.com/2021/07/aspnet-core-6-and-iasyncenumerable.html.