The anatomy of async iterators (aka await, foreach, yield)

Here I'm going to discuss the mechanisms and concepts relating to async iterators in C# - with the hope of both demystifying them a bit, and also showing how we can use some of the more advanced (but slightly hidden) features. I'm going to give some illustrations of what happens under the hood, but note: these are illustrations, not the literal generated expansion - this is deliberately to help show what is conceptually happening, so if I ignore some sublte implementation detail: that's not accidental. As always, if you want to see the actual code, tools like https://sharplab.io/ are awesome (just change the "Results" view to "C#" and paste the code you're interested in onto the left).

Iterators in the sync world

Before we discuss async iterators, let's start by recapping iterators. Many folks may already be familiar with all of this, but hey: it helps to set the scene. More importantly, it is useful to allow us to compare and contrast later when we look at how async changes things. So: we know that we can write a foreach loop (over a sequence) of the form:

foreach (var item in SomeSource(42))  {      Console.WriteLine(item);  }

and for each item that SomeSource returns, we'll get a line in the console. SomeSource could be returning a fully buffered set of data (like a List<string>):

IEnumerable<string> SomeSource(int x)  {      var list = new List<string>();      for (int i = 0; i < 5; i++)          list.Add($"result from SomeSource, x={x}, result {i}");      return list;  }

but a problem here is that this requires SomeSource to run to completion before we get even the first result, which could take a lot of time and memory - and is just generally restrictive. Often, when we're trying to represent a sequence, it may be unbounded, or at least: open-ended - for example, we could be pulling data from a remote work queue, where a: we only want to be holding one pending item at a time, and b: it may not have a logical "end". It turns out that C#'s definition of a "sequence" (for the purposes of foreach) is fine with this. Instead of returning a list, we can write an iterator block:

IEnumerable<string> SomeSource(int x)  {      for (int i = 0; i < 5; i++)          yield return $"result from SomeSource, x={x}, result {i}";  }

This works similarly, but there are some fundamental differences - most noticeably: we don't ever have a buffer - we just make one element available at a time. To understand how this can work, it is useful to take another look at our foreach; the compiler interprets foreach as something like the following:

using (var iter = SomeSource(42).GetEnumerator())  {      while (iter.MoveNext())      {          var item = iter.Current;          Console.WriteLine(item);      }  }

We have to be a little loose in our phrasing here, because foreach isn't actually tied to IEnumerable<T> - it is duck-typed against an API shape instead; the using may or may not be there, for example. But fundamentally, the compiler calls GetEnumerator() on the expression passed to foreach, then creates a while loop checking MoveNext() (which defines "is there more data?" and advances the mechanism in the success case), then accesses the Current property (which exposes the element we advanced to). As an aside, historically (prior to C# 5) the compiler used to scope item outside of the while loop, which might sound innocent, but it was the source of absolutely no end of confusion, code erros, and questions on Stack Overflow (think "captured variables").

So; hopefully you can see in the above how the consumer can access an unbounded forwards-only sequence via this MoveNext() / Current approach; but how does that get implemented? Iterator blocks (anything involving the yield keyword) are actually incredibly complex, so I'm going to take a lot of liberties here, but what is going on is similar to:

IEnumerable<string> SomeSource(int x)      => new GeneratedEnumerable(x);    class GeneratedEnumerable : IEnumerable<string>  {      private int x;      public GeneratedEnumerable(int x)          => this.x = x;        public IEnumerator<string> GetEnumerator()          => new GeneratedEnumerator(x);        // non-generic fallback      IEnumerator IEnumerable.GetEnumerator()          => GetEnumerator();  }    class GeneratedEnumerator : IEnumerator<string>  {      private int x, i;      public GeneratedEnumerator(int x)          => this.x = x;        public string Current { get; private set; }        // non-generic fallback      object IEnumerator.Current => Current;        // if we had "finally" code, it would go here      public void Dispose() { }        // our "advance" logic      public bool MoveNext()      {          if (i < 5)          {              Current = $"result from SomeSource, x={x}, result {i}";              i++;              return true;          }          else          {              return false;          }      }        // this API is essentially deprecated and never used      void IEnumerator.Reset() => throw new NotSupportedException();  }

Let's tear this apart:

• firstly, we need some object to represent IEnumerable<T>, but we also need to understand that IEnumerable<T> and IEnumerator<T> (as returned from GetEnumerator()) are different APIs; in the generated version there is a lot of overlap and they can share an instance, but to help discuss it, I've kept the two concepts separate.
• when we call SomeSource, we create our GeneratedEnumerable which stores the state (x) that was passed to SomeSource, and exposes the required IEnumerable<T> API
• later (and it could be much later), when the caller iterates (foreach) the data, GetEnumerator() is invoked, which calls into our GeneratedEnumerator to act as the cursor over the data
• our MoveNext() logic implements the same for loop conceptually, but one step per call to MoveNext(); if there is more data, Current is assigned with the thing we would have passed to yield return
• note that there is also a yield break C# keyword, which terminates iteration; this would essentially be return false in the generated expansion
• note that there are some nuanced differences in my hand-written version that the C# compiler needs to deal with; for example, what happens if I change x in my enumerator code (MoveNext()), and then later iterate the data a second time - what is the value of x? emphasis: I don't care about this nuance for this discussion!

Hopefully this gives enough of a flavor to understand foreach and iterators (yield) - now let's get onto the more interesting bit: async.

Why do we need async iterators?

The above works great in a synchronous world, but a lot of .NET work is now favoring async/await, in particular to improve server scalability. The big problem in the above code is the bool MoveNext(). This is explicitly synchronous. If the thing it is doing takes some time, we'll be blocking a thread, and blocking a thread is increasingly anathema to us. In the context of our earlier "remote work queue" example, there might not be anything there for seconds, minutes, hours. We really don't want to block threads for that kind of time! The closest we can do without async iterators is to fetch the data asynchronously, but buffered - for example:

async Task<List<string>> SomeSource(int x) {...}

But this is not the same semantics - and is getting back into buffering. Assuming we don't want to fetch everything in one go, to get around this we'd eventually end up implementing some kind of "async batch loop" monstrosity that effectily re-implements foreach using manual ugly code, negating the reasons that foreach even exists. To address this, C# and the BCL have recently added support for async iterators, yay! The new APIs (which are available down to net461 and netstandard20 via NuGet) are:

public interface IAsyncEnumerable<out T>  {      IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default);  }  public interface IAsyncEnumerator<out T> : IAsyncDisposable  {      T Current { get; }      ValueTask<bool> MoveNextAsync();  }  public interface IAsyncDisposable  {      ValueTask DisposeAsync();  }

Let's look at our example again, this time: with added async; we'll look at the consumer first (the code doing the foreach), so for now, let's imagine that we have:

IAsyncEnumerable<string> SomeSourceAsync(int x)      => throw new NotImplementedException();

and focus on the loop; C# now has the await foreach concept, so we can do:

await foreach (var item in SomeSourceAsync(42))  {      Console.WriteLine(item);  }

and the compiler interprets this as something similar to:

await using (var iter = SomeSourceAsync(42).GetAsyncEnumerator())  {      while (await iter.MoveNextAsync())      {          var item = iter.Current;          Console.WriteLine(item);      }  }

(note that await using is similar to using, but DisposeAsync() is called and awaited, instead of Dispose() - even cleanup code can be asynchronous!)

The key point here is that this is actually pretty similar to our sync version, just with added await. Ultimately, however, the moment we add await the entire body is ripped apart by the compiler and rewritten as an asynchronous state machine. That isn't the topic of this article, so I'm not even going to try and cover how await is implemented behind the scenes. For today "a miracle happens" will suffice for that. The observant might also be wondering "wait, but what about cancellation?" - don't worry, we'll get there!

So what about our enumerator? Along with await foreach, we can also now write async iterators with yield; for example, we could do:

async IAsyncEnumerable<string> SomeSourceAsync(int x)  {      for (int i = 0; i < 5; i++)      {          await Task.Delay(100); // simulate async something          yield return $"result from SomeSource, x={x}, result {i}";      }  }

In real code, we could now be consuming data from a remote source asynchronously, and we have a very effective mechanism for expressing open sequences of asynchronous data. In particular, remember that the await iter.MoveNextAsync() might complete synchronously, so if data is available immediately, there is no context switch. We can imagine, for example, an iterator block that requests data from a remote server in pages, and yield return each record of the data in the current page (making it available immediately), only doing an await when it needs to fetch the next page.

Behind the scenes, the compiler generates types to implement the IAsyncEnumerable<T> and IAsyncEnumerator<T> pieces, but this time they are even more obtuse, owing to the async/await restructuring. I do not intend to try and cover those here - it is my hope instead that we wave a hand and say "you know that expansion we wrote by hand earlier? like that, but with more async". However, there is a very important topic that we have overlooked, and that we should cover: cancellation.

But what about cancellation?

Most async APIs support cancellation via a CancellationToken, and this is no exception; look back up to IAsyncEnumerable<T> and you'll see that it can be passed into the GetAsyncEnumerator() method. But if we're not writing the loop by hand, how do we do this? This is achieved via WithCancellation, similarly do how ConfigureAwait can be used to configure await - and indeed, there's even a ConfigureAwait we can use too! For example, we could do (showing both config options in action here):

await foreach (var item in SomeSourceAsync(42)      .WithCancellation(cancellationToken).ConfigureAwait(false))  {      Console.WriteLine(item);  }

which would be semantically equivalent to:

var iter = SomeSourceAsync(42).GetAsyncEnumerator(cancellationToken);  await using (iter.ConfigureAwait(false))  {      while (await iter.MoveNextAsync().ConfigureAwait(false))      {          var item = iter.Current;          Console.WriteLine(item);      }  }

(I've had to split the iter local out to illustrate that the ConfigureAwait applies to the DisposeAsync() too - via await iter.DisposeAsync().ConfigureAwait(false) in a finally)

So; now we can pass a CancellationToken into our iterator... but - how can we use it? That's where things get even more fun! The naive way to do this would be to think along the lines of "I can't take a CancellationToken until GetAsyncEnumerator is called, so... perhaps I can create a type to hold the state until I get to that point, and create an iterator block on the GetAsyncEnumerator method" - something like:

// this is unnecessary; do not copy this!  IAsyncEnumerable<string> SomeSourceAsync(int x)      => new SomeSourceEnumerable(x);  class SomeSourceEnumerable : IAsyncEnumerable<string>  {      private int x;      public SomeSourceEnumerable(int x)          => this.x = x;        public async IAsyncEnumerator<string> GetAsyncEnumerator(          CancellationToken cancellationToken = default)      {          for (int i = 0; i < 5; i++)          {              await Task.Delay(100, cancellationToken); // simulate async something              yield return $"result from SomeSource, x={x}, result {i}";          }      }  }

The above works. If a CancellationToken is passed in via WithCancellation, our iterator will be cancelled at the correct time - including during the Task.Delay; we could also check IsCancellationRequested or call ThrowIfCancellationRequested() at any point in our iterator block, and all the right things would happen. But; we're making life hard for ourselves - the compiler can do this for us, via [EnumeratorCancellation]. We could also just have:

async IAsyncEnumerable<string> SomeSourceAsync(int x,      [EnumeratorCancellation] CancellationToken cancellationToken = default)  {      for (int i = 0; i < 5; i++)      {          await Task.Delay(100, cancellationToken); // simulate async something          yield return $"result from SomeSource, x={x}, result {i}";      }  }

This works similarly to our approach above - our cancellationToken parameter makes the token from GetAsyncEnumerator() (via WithCancellation) available to our iterator block, and we haven't had to create any dummy types. There is one slight nuance, though... we've changed the signature of SomeSourceAsync by adding a parameter. The code we had above still compiles because the parameter is optional. But this prompts the question: what happens if I passed one in? For example, what are the differences between:

// option A - no cancellation  await foreach (var item in SomeSourceAsync(42))    // option B - cancellation via WithCancellation  await foreach (var item in SomeSourceAsync(42).WithCancellation(cancellationToken))    // option C - cancellation via SomeSourceAsync  await foreach (var item in SomeSourceAsync(42, cancellationToken))    // option D - cancellation via both  await foreach (var item in SomeSourceAsync(42, cancellationToken).WithCancellation(cancellationToken))    // option E - cancellation via both with different tokens  await foreach (var item in SomeSourceAsync(42, tokenA).WithCancellation(tokenB))

The answer is that the right thing happens: it doesn't matter which API you use - if a cancellation token is provided, it will be respected. If you pass two different tokens, then when either token is cancelled, it will be considered cancelled. What happens is that the original token passed via the parameter is stored as a field on the generated enumerable type, and when GetAsyncEnumerator is called, the parameter to GetAsyncEnumerator and the field are inspected. If they are both genuine but different cancellable tokens, CancellationTokenSource.CreateLinkedTokenSource is used to create a combined token (you can think of CreateLinkedTokenSource as the cancellation version of Task.WhenAny); otherwise, if either is genuine and cancellable, it is used. The result is that when you write an async cancellable iterator, you don't need to worry too much about whether the caller used the API directly vs indirectly.

You might be more concerned by the fact that we've changed the signature, however; in that case, a neat trick is to use two methods - one without the token that is for consumers, and one with the token for the actual implementation:

public IAsyncEnumerable<string> SomeSourceAsync(int x)      => SomeSourceImplAsync(x);    private async IAsyncEnumerable<string> SomeSourceImplAsync(int x,      [EnumeratorCancellation] CancellationToken cancellationToken = default)  {      for (int i = 0; i < 5; i++)      {          await Task.Delay(100, cancellationToken); // simulate async something          yield return $"result from SomeSource, x={x}, result {i}";      }  }

This would seem an ideal candidate for a "local function", but unfortunately at the current time, parameters on local functions are not allowed to be decorated with attributes. It is my hope that the language / compiler folks take pity on us, and allow us to do (in the future) something more like:

public IAsyncEnumerable<string> SomeSourceAsync(int x)  {      return Impl();        // this does not compile today      async IAsyncEnumerable<string> Impl(          [EnumeratorCancellation] CancellationToken cancellationToken = default)      {          for (int i = 0; i < 5; i++)          {              await Task.Delay(100, cancellationToken); // simulate async something              yield return $"result from SomeSource, x={x}, result {i}";          }      }  }

or the equivalent using static local functions, which is usually my preference to avoid any surprises in how capture works. The good news is that this works in the preview language versions, but that is not a guarantee that it will "land".

Summary

So; that's how you can implement and use async iterators in C# now. We've looked at both the consumer and producer versions of iterators, for both synchronous and asynchronous code paths, and looked at various ways of accessing cancellation of asynchronous iterators. There is a lot going on here, but: hopefully it is useful and meaningful.

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