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async-streams (C# 8.0)

Async-streams are asynchronous variants of enumerables, where getting the next element may involve an asynchronous operation. They are types that implement IAsyncEnumerable<T>.

// Those interfaces will ship as part of .NET Core 3
namespace System.Collections.Generic
{
    public interface IAsyncEnumerable<out T>
    {
        IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken token = default);
    }

    public interface IAsyncEnumerator<out T> : System.IAsyncDisposable
    {
        System.Threading.Tasks.ValueTask<bool> MoveNextAsync();
        T Current { get; }
    }
}
namespace System
{
    public interface IAsyncDisposable
    {
        System.Threading.Tasks.ValueTask DisposeAsync();
    }
}

When you have an async-stream, you can enumerate its items using an asynchronous foreach statement: await foreach (var item in asyncStream) { ... }. An await foreach statement is just like a foreach statement, but it uses IAsyncEnumerable instead of IEnumerable, each iteration evaluates an await MoveNextAsync(), and the disposable of the enumerator is asynchronous.

Similarly, if you have an async-disposable, you can use and dispose it with asynchronous using statement: await using (var resource = asyncDisposable) { ... } An await using statement is just like a using statement, but it uses IAsyncDisposable instead of IDisposable, and await DisposeAsync() instead of Dispose().

The user can implement those interfaces manually, or can take advantage of the compiler generating a state-machine from a user-defined method (called an "async-iterator" method). An async-iterator method is a method that:

  1. is declared async,
  2. returns an IAsyncEnumerable<T> or IAsyncEnumerator<T> type,
  3. uses both await syntax (await expression, await foreach or await using statements) and yield statements (yield return, yield break).

For example:

async IAsyncEnumerable<int> GetValuesFromServer()
{
    while (true)
    {
        IEnumerable<int> batch = await GetNextBatch();
        if (batch == null) yield break;

        foreach (int item in batch)
        {
            yield return item;
        }
    }
}

Just like in iterator methods, there are several restrictions on where a yield statement can appear in async-iterator methods:

  • It is a compile-time error for a yield statement (of either form) to appear in the finally clause of a try statement.
  • It is a compile-time error for a yield return statement to appear anywhere in a try statement that contains any catch clauses.

Detailed design for await using statement

An asynchronous using is lowered just like a regular using, except that Dispose() is replaced with await DisposeAsync().

Note that pattern-based lookup for DisposeAsync binds to instance methods that can be invoked without arguments. Extension methods do not contribute. The result of DisposeAsync must be awaitable.

Detailed design for await foreach statement

An await foreach is lowered just like a regular foreach, except that:

  • GetEnumerator() is replaced with await GetAsyncEnumerator()
  • MoveNext() is replaced with await MoveNextAsync()
  • Dispose() is replaced with await DisposeAsync()

Note that pattern-based lookup for GetAsyncEnumerator, MoveNextAsync and DisposeAsync binds to instance methods that can be invoked without arguments. Extension methods do not contribute. The result of MoveNextAsync and DisposeAsync must be awaitable. Disposal for await foreach does not include a fallback to a runtime check for an interface implementation.

Asynchronous foreach loops are disallowed on collections of type dynamic, as there is no asynchronous equivalent of the non-generic IEnumerable interface.

But wrapper types can pass non-default values (see .WithCancellation(CancellationToken) extension method), thereby allowing consumers of async-streams to control cancellation. A producer of async-streams can make use of the cancellation token by writing an IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken) async-iterator method in a custom type.

E e = ((C)(x)).GetAsyncEnumerator(default);
try
{
    while (await e.MoveNextAsync())
    {
        V v = (V)(T)e.Current;  -OR-  (D1 d1, ...) = (V)(T)e.Current;
        // body
    }
}
finally
{
    await e.DisposeAsync();
}

Detailed design for async-iterator methods

An async-iterator method is replaced by a kick-off method, which initializes a state machine. It does not start running the state machine (unlike kick-off methods for regular async method). The kick-off method method is marked with AsyncIteratorStateMachineAttribute.

The state machine for an enumerable async-iterator method primarily implements IAsyncEnumerable<T> and IAsyncEnumerator<T>. For an enumerator async-iterator, it only implements IAsyncEnumerator<T>. It is similar to a state machine produced for an async method. It contains builder and awaiter fields, used to run the state machine in the background (when an await is reached in the async-iterator). It also captures parameter values (if any) or this (if needed).

But it contains additional state:

  • a promise of a value-or-end,
  • a current yielded value of type T,
  • an int capturing the id of the thread that created it,
  • a bool flag indicating "dispose mode".

The central method of the state machine is MoveNext(). It gets run by MoveNextAsync(), or as a background continuation initiated from these from an await in the method.

The promise of a value-or-end is returned from MoveNextAsync. It can be fulfilled with either:

  • true (when a value becomes available following background execution of the state machine),
  • false (if the end is reached),
  • an exception. The promise is implemented as a ManualResetValueTaskSourceCore<bool> (which is a re-usable and allocation-free way of producing and fulfilling ValueTask<bool> or ValueTask instances) and its surrounding interfaces on the state machine: IValueTaskSource<bool> and IValueTaskSource. See more details about those types at https://blogs.msdn.microsoft.com/dotnet/2018/11/07/understanding-the-whys-whats-and-whens-of-valuetask/

Compared to the state machine for a regular async method, the MoveNext() for an async-iterator method adds logic:

  • to support handling a yield return statement, which saves the current value and fulfills the promise with result true,
  • to support handling a yield break statement, which sets the dispose mode on and jumps to the enclosing finally or exit,
  • to dispatch execution to finally blocks (when disposing),
  • to exit the method, which fulfills the promise with result false,
  • to catch exceptions, which set the exception into the promise. (The handling of an await is unchanged)

This is reflected in the implementation, which extends the lowering machinery for async methods to:

  1. handle yield return and yield break statements (see methods VisitYieldReturnStatement and VisitYieldBreakStatement to AsyncIteratorMethodToStateMachineRewriter),
  2. handle try statements (see methods VisitTryStatement and VisitExtractedFinallyBlock in AsyncIteratorMethodToStateMachineRewriter)
  3. produce additional state and logic for the promise itself (see AsyncIteratorRewriter, which produces various other members: MoveNextAsync, Current, DisposeAsync, and some members supporting the resettable ValueTask behavior, namely GetResult, SetStatus, OnCompleted).
ValueTask<bool> MoveNextAsync()
{
    if (state == StateMachineStates.FinishedStateMachine)
    {
        return default(ValueTask<bool>);
    }
    valueOrEndPromise.Reset();
    var inst = this;
    builder.Start(ref inst);
    var version = valueOrEndPromise.Version;
    if (valueOrEndPromise.GetStatus(version) == ValueTaskSourceStatus.Succeeded)
    {
        return new ValueTask<bool>(valueOrEndPromise.GetResult(version));
    }
    return new ValueTask<bool>(this, version); // note this leverages the state machine's implementation of IValueTaskSource<bool>
}
T Current => current;

The kick-off method and the initialization of the state machine for an async-iterator method follows those for regular iterator methods. In particular, the synthesized GetAsyncEnumerator() method is like GetEnumerator() except that it sets the initial state to to StateMachineStates.NotStartedStateMachine (-1):

IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken token)
{
    {StateMachineType} result;
    if (initialThreadId == /*managedThreadId*/ && state == StateMachineStates.FinishedStateMachine)
    {
        state = InitialState; // -3
        disposeMode = false;
        result = this;
    }
    else
    {
        result = new {StateMachineType}(InitialState);
    }
    /* copy all of the parameter proxies */
}

For a discussion of the threadID check, see https://github.com/dotnet/corefx/issues/3481

Similarly, the kick-off method is much like those of regular iterator methods:

{
    {StateMachineType} result = new {StateMachineType}(StateMachineStates.FinishedStateMachine); // -2
    /* save parameters into parameter proxies */
    return result;
}

Disposal

Iterator and async-iterator methods need disposal because their execution steps are controlled by the caller, which could choose to dispose the enumerator before getting all of its elements. For example, foreach (...) { if (...) break; }. In contrast, async methods continue running autonomously until they are done. They are never left suspended in the middle of execution from the caller's perspective, so they don't need to be disposed.

In summary, disposal of an async-iterator works based on four design elements:

  • yield return (jumps to finally when resuming in dispose mode)
  • yield break (enters dispose mode and jumps to enclosing finally)
  • finally (after a finally we jump to the next enclosing one)
  • DisposeAsync (enters dispose mode and resumes execution)

The caller of an async-iterator method should only call DisposeAsync() when the method completed or was suspended by a yield return. DisposeAsync sets a flag on the state machine ("dispose mode") and (if the method wasn't completed) resumes the execution from the current state. The state machine can resume execution from a given state (even those located within a try). When the execution is resumed in dispose mode, it jumps straight to the enclosing finally. finally blocks may involve pauses and resumes, but only for await expressions. As a result of the restrictions imposed on yield return (described above), dispose mode never runs into a yield return. Once a finally block completes, the execution in dispose mode jumps to the next enclosing finally, or the end of the method once we reach the top-level.

Reaching a yield break also sets the dispose mode flag and jumps to the enclosing finally (or end of the method). By the time we return control to the caller (completing the promise as false by reaching the end of the method) all disposal was completed, and the state machine is left in finished state. So DisposeAsync() has no work left to do.

Looking at disposal from the perspective of a given finally block, the code in that block can get executed:

  • by normal execution (ie. after the code in the try block),
  • by raising an exception inside the try block (which will execute the necessary finally blocks and terminate the method in Finished state),
  • by calling DisposeAsync() (which resumes execution in dispose mode and jumps to the enclosing finally),
  • following a yield break (which enters dispose mode and jumps to the enclosing finally),
  • in dispose mode, following a nested finally.

A yield return is lowered as:

_current = expression;
_state = <next_state>;
goto <exprReturnTruelabel>; // which does _valueOrEndPromise.SetResult(true); return;

// resuming from state=<next_state> will dispatch execution to this label
<next_state_label>: ;
this.state = cachedState = NotStartedStateMachine;
if (disposeMode) /* jump to enclosing finally or exit */

A yield break is lowered as:

disposeMode = true;
/* jump to enclosing finally or exit */
ValueTask IAsyncDisposable.DisposeAsync()
{
    if (state >= StateMachineStates.NotStartedStateMachine /* -1 */)
    {
        throw new NotSupportedException();
    }
    if (state == StateMachineStates.FinishedStateMachine /* -2 */)
    {
        return default;
    }
    disposeMode = true;
    _valueOrEndPromise.Reset();
    var inst = this;
    _builder.Start(ref inst);
    return new ValueTask(this, _valueOrEndPromise.Version);  // note this leverages the state machine's implementation of IValueTaskSource
}
Regular versus extracted finally

When the finally clause contains no await expressions, a try/finally is lowered as:

try
{
    ...
    finallyEntryLabel:
}
finally
{
    ...
}
if (disposeMode) /* jump to enclosing finally or exit */

When a finally contains await expressions, it is extracted before async rewriting (by AsyncExceptionHandlerRewriter). In those cases, we get:

try
{
    ...
    goto finallyEntryLabel;
}
catch (Exception e)
{
    ... save exception ...
}
finallyEntryLabel:
{
    ... original code from finally and additional handling for exception ...
}

In both cases, we will add a if (disposeMode) /* jump to enclosing finally or exit */ after the block for finally logic.

State values and transitions

The enumerable starts with state -2. Calling GetAsyncEnumerator sets the state to -3, or returns a fresh enumerator (also with state -3).

From there, MoveNext will either:

  • reach the end of the method (-2, we're done and disposed)
  • reach a yield break (state unchanged, dispose mode = true)
  • reach a yield return (-N, decreasing from -4)
  • reach an await (N, increasing from 0)

From suspended state N or -N, MoveNext will resume execution (-1). But if the suspension was a yield return (-N), you could also call DisposeAsync, which resumes execution (-1) in dispose mode.

When in dispose mode, MoveNext continues to suspend (N) and resume (-1) until the end of the method is reached (-2).

The result of invoking DisposeAsync from states -1 or N is unspecified. This compiler generates throw new NotSupportException() for those cases.

        DisposeAsync                              await
 +------------------------+             +------------------------> N
 |                        |             |                          |
 v   GetAsyncEnumerator   |             |        resuming          |
-2 --------------------> -3 --------> -1 <-------------------------+    Dispose mode = false
 ^                                   |  |                          |
 |         done and disposed         |  |      yield return        |
 +-----------------------------------+  +-----------------------> -N
 |                                   |                             |
 |                                   |                             |
 |                             yield |                             |
 |                             break |           DisposeAsync      |
 |                                   |  +--------------------------+
 |                                   |  |
 |                                   |  |
 |         done and disposed         v  v    suspension (await)
 +----------------------------------- -1 ------------------------> N
                                        ^                          |    Dispose mode = true
                                        |         resuming         |
                                        +--------------------------+
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