csharplang/proposals/async-method-builders.md

Per-Method AsyncMethodBuilders

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Summary

Extend the existing async method builder to support attribution per-method in addition to the existing per-return type support.

Motivation

Today, async method builders are tied to a given type used as a return type of an async method. For example, any method that's declared as async Task uses AsyncTaskMethodBuilder, and any method that's declared as async ValueTask<T> uses AsyncValueTaskMethodBuilder<T>. This is due to the [AsyncMethodBuilder(Type)] attribute on the type used as a return type, e.g. ValueTask<T> is attributed as [AsyncMethodBuilder(typeof(AsyncValueTaskMethodBuilder<>))]. This addresses the majority common case, but it leaves a few notable holes for advanced scenarios.

In .NET 5, an experimental feature was shipped that provides two modes in which AsyncValueTaskMethodBuilder and AsyncValueTaskMethodBuilder<T> operate. The on-by-default mode is the same as has been there since the functionality was introduced: when the state machine needs to be lifted to the heap, an object is allocated to store the state, and the async method returns a ValueTask{<T>} backed by a Task{<T>}. However, if an environment variable is set, all builders in the process switch to a mode where, instead, the ValueTask{<T>} instances are backed by reusable IValueTaskSource{<T>} implementations that are pooled. Each async method has its own pool with a fixed maximum number of instances allowed to be pooled, and as long as no more than that number are ever returned to the pool to be pooled at the same time, async ValueTask<{T}> methods effectively become free of any GC allocation overhead.

There are several problems with this experimental mode, however, which is both why a) it's off by default and b) we're likely to remove it in a future release unless very compelling new information emerges (https://github.com/dotnet/runtime/issues/13633).

• It introduces a behavioral difference for consumers of the returned ValueTask{<T>} if that ValueTask isn't being consumed according to spec. When it's backed by a Task, you can do with the ValueTask things you can do with a Task, like await it multiple times, await it concurrently, block waiting for it to complete, etc. But when it's backed by an arbitrary IValueTaskSource, such operations are prohibited, and automatically switching from the former to the latter can lead to bugs. With the switch at the process level and affecting all async ValueTask methods in the process, whether you control them or not, it's too big a hammer.
• It's not necessarily a performance win, and could represent a regression in some situations. The implementation is trading the cost of pooling (accessing a pool isn't free) with the cost of GC, and in various situations the GC can win. Again, applying the pooling to all async ValueTask methods in the process rather than being selective about the ones it would most benefit is too big a hammer.
• It adds to the IL size of a trimmed application, even if the flag isn't set, and then to the resulting asm size. It's possible that can be worked around with improvements to the implementation to teach it that for a given deployment the environment variable will always be false, but as it stands today, every async ValueTask method saw for example an ~2K binary footprint increase in aot images due to this option, and, again, that applies to all async ValueTask methods in the whole application closure.
• Different methods may benefit from differing levels of control, e.g. the size of the pool employed because of knowledge of the method and how it's used, but the same setting is applied to all uses of the builder. One could imagine working around that by having the builder code use reflection at runtime to look for some attribute, but that adds significant run-time expense, and likely on the startup path.

On top of all of these issues with the existing pooling, it's also the case that developers are prevented from writing their own customized builders for types they don't own. If, for example, a developer wants to implement their own pooling support, they also have to introduce a brand new task-like type, rather than just being able to use {Value}Task{<T>}, because the attribute specifying the builder is only specifiable on the type declaration of the return type.

We need a way to have an individual async method opt-in to a specific builder.

Detailed design

P0: AsyncMethodBuilderAttribute applicable to methods

In dotnet/runtime, change AsyncMethodBuilderAttribute's AttributeUsage from:

AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.Delegate | AttributeTargets.Enum

to also include Method:

AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.Delegate | AttributeTargets.Enum | AttributeTargets.Method

so that it may be applied to methods as well. (Alternatively, introduce a new attribute specific to methods, if that's deemed better for some reason.)

In the C# compiler, prefer the attribute on the method when determining what builder to use over the one defined on the type. For example, today if a method is defined as:

public async ValueTask<T> ExampleAsync() { ... }

the compiler will generate code akin to:

[AsyncStateMachine(typeof(<ExampleAsync>d__29))]
[CompilerGenerated]
static ValueTask<int> ExampleAsync()
{
	<ExampleAsync>d__29 stateMachine;
	stateMachine.<>t__builder = AsyncValueTaskMethodBuilder<int>.Create();
	stateMachine.<>1__state = -1;
	stateMachine.<>t__builder.Start(ref stateMachine);
	return stateMachine.<>t__builder.Task;
}

With this change, if the developer wrote:

[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder<int>))] // new, referring to some custom builder type
static async ValueTask<int> ExampleAsync() { ... }

it would instead be compiled to:

[AsyncStateMachine(typeof(<ExampleAsync>d__29))]
[CompilerGenerated]
[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder<int>))] // retained but not necessary anymore
static ValueTask<int> ExampleAsync()
{
	<ExampleAsync>d__29 stateMachine;
	stateMachine.<>t__builder = PoolingAsyncValueTaskMethodBuilder<int>.Create(); // <>t__builder now a different type
	stateMachine.<>1__state = -1;
	stateMachine.<>t__builder.Start(ref stateMachine);
	return stateMachine.<>t__builder.Task;
}

Just those small additions enable:

• Anyone to write their own builder that can be applied to async methods that return Task{T} and ValueTask{<T>}
• As "anyone", the runtime to ship the experimental builder support as new public builder types that can be opted into on a method-by-method basis; the existing support would be removed from the existing builders. Methods (including some we care about in the core libraries) can then be attributed on a case-by-case basis to use the pooling support, without impacting any other unattributed methods.

and with minimal surface area changes or feature work in the compiler.

P1: AsyncMethodBuilderAttribute arguments forward to Create

The attribute would be given an additional constructor:

public AsyncMethodBuilderAttribute(Type builderType, params object[] createArguments);

If any such arguments are specified, the compiler would expect the builder to have a Create method that could bind with those arguments, e.g. if a developer used:

[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder<>), 16)]

the compiler would allow compilation due to PoolingAsyncValueTaskMethodBuilder<> exposing the following Create overload:

public static PoolingAsyncValueTaskMethodBuilder<T> Create(int poolCapacity);

and would use that Create overload instead of a parameterless Create that it would otherwise expect and use, e.g. this:

[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder<>), 16)]
static async ValueTask<int> ExampleAsync() { ... }

would be compiled to:

[AsyncStateMachine(typeof(<ExampleAsync>d__29))]
[CompilerGenerated]
[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder<>), 16)]
static ValueTask<int> ExampleAsync()
{
	<ExampleAsync>d__29 stateMachine;
	stateMachine.<>t__builder = PoolingAsyncValueTaskMethodBuilder<int>.Create(16); // attr arguments passed to Create
	stateMachine.<>1__state = -1;
	stateMachine.<>t__builder.Start(ref stateMachine);
	return stateMachine.<>t__builder.Task;
}

Such support would enable custom builders to be parameterized per call site, without requiring the builder to perform complicated and expensive reflection.

P2: Enable at the module (and type?) level as well

A developer that wants to using a specific custom builder for all of their methods can do so by putting the relevant attribute on each method. But we could also enable attributing at the module or type level, in which case every relevant method within that scope would behave as if it were directly annotated, e.g.

[module: PoolingAsyncValueTaskMethodBuilder]
[module: PoolingAsyncValueTaskMethodBuilder<>]

class MyClass
{
    public async ValueTask Method1Async() { ... } // would use PoolingAsyncValueTaskMethodBuilder
    public async ValueTask<int> Method2Async() { ... } // would use PoolingAsyncValueTaskMethodBuilder<int>
    public async ValueTask<string> Method3Async() { ... } // would use PoolingAsyncValueTaskMethodBuilder<string>
}

This would not only make it more convenient than putting the attribute on every method, it would also make it easier to employ different builds that used different builders. For example, a build optimized for throughput might include a .cs file that specifies a pooling builder in a module-level attribute, whereas a build optimized for size might a include a .cs file that specifies a minimalistic builder that opts to use more allocation/boxing instead of lots of generic specialization and throughput optimizations that lead to code bloat.

Drawbacks

• The syntax for applying such an attribute to a method is verbose. This is an advanced feature, but a developer using it frequently could create their own attribute derived from the one they care about, and then use that derived attribute to simplify, e.g.

class Pool<T> : AsyncMethodBuilderAttribute<T>
{
    public Pool() : base(typeof(AsyncValueTaskMethodBuilder<T>)) { }
}
...
[Pool]
internal async ValueTask<int> ExampleAsync() { ... }

I don't know exactly how this could be made to work with the createArguments support, though. The derived attribute could also accept a params array and pass it down to the base, but the compiler would need to be able to recognize it. Maybe a heuristic of special-casing any params object[] at the end of an AsyncMethodBuilder-derived attribute.

Alternatives

• Implement a different task-like type and expose that difference to consumers. ValueTask was made extensible via the IValueTaskSource interface to avoid that need, however.
• Address just the ValueTask pooling part of the issue by enabling the experiment as the on-by-default-and-only implementation. That doesn't address other aspects, such as configuring the pooling, or enabling someone else to provide their own builder.

Unresolved questions

1. Attribute. Should we reuse [AsyncMethodBuilder(typeof(...))] or introduce yet another attribute?
2. Replace or also create. All of the examples in this proposal are about replacing a buildable task-like's builder. Should the feature be scoped to just that? Or should you be able to use this attribute on a method with a return type that doesn't already have a builder (e.g. some common interface)? That could impact overload resolution.
3. Virtuals / Interfaces. What is the behavior if the attribute is specified on an interface method? I think it should either be a nop or a compiler warning/error, but it shouldn't impact implementations of the interface. A similar question exists for base methods that are overridden, and there again I don't think the attribute on the base method should impact how an override implementation behaves. Note the current attribute has Inherited = false on its AttributeUsage.
4. Precedence. If we wanted to do the module/type-level annotation, we would need to decide on which attribution wins in the case where multiple ones applied (e.g. one on the method, one on the containing module). We would also need to determine if this would necessitate using a different attribute (see (1) above), e.g. what would the behavior be if a task-like type was in the same scope? Or if a buildable task-like itself had async methods on it, would they be influenced by the attributed applied to the task-like type to specify its default builder?
5. Private Builders. Should the compiler support non-public async method builders? This is not spec'd today, but experimentally we only support public ones. That makes some sense when the attribute is applied to a type to control what builder is used with that type, since anyone writing an async method with that type as the return type would need access to the builder. However, with this new feature, when that attribute is applied to a method, it only impacts the implementation of that method, and thus could reasonably reference a non-public builder. Likely we will want to support library authors who have non-public ones they want to use.

Design meetings

Link to design notes that affect this proposal, and describe in one sentence for each what changes they led to.