[API Proposal]: Allow method built with an AsyncMethodBuilder to return something else than T

[dr1rrb]

Background and motivation

When you implement a custom "Async Method Builder" to create your own instance of MyTask<T> the single type that the user can return in the function is T.

But, we could perfectly imagine cases where we can create a MyTask<T> with something else than T, for instance a T? (or a custom Option<T>).

My uses case is where MyTask<T> will implement IOtherInterface<GenericType<T>>. Restricting async method builder to only T means either:

• Users will have to write MyTask<GenericType<T>>, which not possible since MyTask must have only one 1 type parameter, we cannot have type constraints on MyTask<T> where T : GenericType<T???>;
• Having an implicit cast from GenericType<T> to T but this will result in a lost of information.

API Proposal

A method that uses an async method builder should accept that the user return any type for which there is a SetResult on the builder.

namespace MyLibrary;

[AsyncMethodBuilder(typeof(MyTaskMethodBuilder<>))]
public class MyTask<T>
{
	public void SetResult(T? value) { }
	public void SetResult(Option<T> value) { }
	public void SetException(Exception error) { }
}

public enum OptionType
{
	None,
	Some,
}

public record struct Option<T>(OptionType Type, T? Value)
{
	public static Option<T> Some(T value) => new(OptionType.Some, value);
	public static Option<T> None() => new(OptionType.None, default);
}

[EditorBrowsable(EditorBrowsableState.Never)]
public readonly struct MyTaskMethodBuilder<T>
{
	private readonly MyTask<T> _task = new();

	public MyTaskMethodBuilder() { }

	public MyTask<T> Task => _task;

	public static FeedMethodBuilder<T> Create()
		=> new();

	public void Start<TStateMachine>(ref TStateMachine stateMachine)
		where TStateMachine : IAsyncStateMachine
		=> stateMachine.MoveNext();

	public void SetStateMachine(IAsyncStateMachine stateMachine) { }

	public void SetException(Exception exception)
		=> _task.SetException(exception);

	public void SetResult(T? result)
		=> _task.SetResult(result);

	public void SetResult(Option<T> result)
		=> _task.SetResult(result);

	public void AwaitOnCompleted<TAwaiter, TStateMachine>(
		ref TAwaiter awaiter, ref TStateMachine stateMachine)
		where TAwaiter : INotifyCompletion
		where TStateMachine : IAsyncStateMachine
		=> awaiter.OnCompleted(stateMachine.MoveNext);

	public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(
		ref TAwaiter awaiter, ref TStateMachine stateMachine)
		where TAwaiter : ICriticalNotifyCompletion
		where TStateMachine : IAsyncStateMachine
		=> awaiter.OnCompleted(stateMachine.MoveNext);
}

API Usage

namespace MyUserApp;

public class MyUserClass
{
	public async MyTask<int> WhenReturnInt() => 42;
	public async MyTask<int> WhenReturnNullableInt() => default(int?);
	public async MyTask<int> WhenReturnOption() => Option<int>.None();
}

Alternative Designs

Two alternatives are possible:

1. Instead of supporting "any type for which there is a SetResult on the builder", we could support only one type (e.g. SetResult(Option<T> result) - as an implicit cast from T to Option<T> is possible here).
2. An acceptable solution for my case, would be to allow nullable value if the SetResult accepts it (i.e. SetResult(T? result)).

Risks

• Allowing a method to return multiple types might be confusing
• Might cause some issue with type inference

[ghost]

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Issue Details

---

Background and motivation

When you implement a custom "Async Method Builder" to create your own instance of MyTask<T> the single type that the user can return in the function is T.

But, we could perfectly imagine cases where we can create a MyTask<T> with something else than T, for instance a T? (or a custom Option<T>).

My uses case is where MyTask<T> will implement IOtherInterface<GenericType<T>>. Restricting async method builder to only T means either:

• Users will have to write MyTask<GenericType<T>>, which not possible since MyTask must have only one 1 type parameter, we cannot have type constraints on MyTask<T> where T : GenericType<T???>;
• Having an implicit cast from GenericType<T> to T but this will result in a lost of information.

API Proposal

A method that uses an async method builder should accept that the user return any type for which there is a SetResult on the builder.

namespace MyLibrary;

[AsyncMethodBuilder(typeof(MyTaskMethodBuilder<>))]
public class MyTask<T>
{
	public void SetResult(T? value) { }
	public void SetResult(Option<T> value) { }
	public void SetException(Exception error) { }
}

public enum OptionType
{
	None,
	Some,
}

public record struct Option<T>(OptionType Type, T? Value)
{
	public static Option<T> Some(T value) => new(OptionType.Some, value);
	public static Option<T> None() => new(OptionType.None, default);
}

[EditorBrowsable(EditorBrowsableState.Never)]
public readonly struct MyTaskMethodBuilder<T>
{
	private readonly MyTask<T> _task = new();

	public MyTaskMethodBuilder() { }

	public MyTask<T> Task => _task;

	public static FeedMethodBuilder<T> Create()
		=> new();

	public void Start<TStateMachine>(ref TStateMachine stateMachine)
		where TStateMachine : IAsyncStateMachine
		=> stateMachine.MoveNext();

	public void SetStateMachine(IAsyncStateMachine stateMachine) { }

	public void SetException(Exception exception)
		=> _task.SetException(exception);

	public void SetResult(T? result)
		=> _task.SetResult(result);

	public void SetResult(Option<T> result)
		=> _task.SetResult(result);

	public void AwaitOnCompleted<TAwaiter, TStateMachine>(
		ref TAwaiter awaiter, ref TStateMachine stateMachine)
		where TAwaiter : INotifyCompletion
		where TStateMachine : IAsyncStateMachine
		=> awaiter.OnCompleted(stateMachine.MoveNext);

	public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(
		ref TAwaiter awaiter, ref TStateMachine stateMachine)
		where TAwaiter : ICriticalNotifyCompletion
		where TStateMachine : IAsyncStateMachine
		=> awaiter.OnCompleted(stateMachine.MoveNext);
}

API Usage

namespace MyUserApp;

public class MyUserClass
{
	public async MyTask<int> WhenReturnInt() => 42;
	public async MyTask<int> WhenReturnNullableInt() => default(int?);
	public async MyTask<int> WhenReturnOption() => Option<int>.None();
}

Alternative Designs

Two alternatives are possible:

1. Instead of supporting "any type for which there is a SetResult on the builder", we could support only one type (e.g. SetResult(Option<T> result) - as an implicit cast from T to Option<T> is possible here).
2. An acceptable solution for my case, would be to allow nullable value if the SetResult accepts it (i.e. SetResult(T? result)).

Risks

• Allowing a method to return multiple types might be confusing
• Might cause some issue with type inference

Author:	dr1rrb
Assignees:	-
Labels:	api-suggestion, area-System.Threading.Tasks, untriaged
Milestone:	-

[teo-tsirpanis]

This seems like a C# language feature. You should open an issue in https://github.com/dotnet/csharplang instead.

[dr1rrb]

Thanks @teo-tsirpanis, I was not sure but since I found the AsyncMethdoBuilderAttribute on this repo I opened it here. But you're right and actually this issue is a duplicate of dotnet/csharplang#3723