Custom async IAsyncEnumerable type?

[timcassell]

C# 7.2 introduced builders for custom task-like types, then C# 8 introduced async enumerators. I'm wondering if it's possible to back an async IAsyncEnumerable<T> function with a custom type in a similar manner, and if so, how?

If I create a type that implements IAsyncEnumerable, await foreach works to consume it. Can that then be used as the return type for the async iterator function with another special builder?

public struct MyAsyncEnumerable<T> : IAsyncEnumerable<T>
{
    ...
}

public async MyAsyncEnumerable<int> EnumerateMyValuesAsync()
{
    await Task.Yield();
    yield return 42;
}

[timcassell]

@stephentoub I saw you wrote the Microsoft article about async enumerables in November 2019, do you have the answer to this?

[stephentoub]

I'm wondering if it's possible to back an async IAsyncEnumerable function with a custom type in a similar manner

It's not.

Can that then be used as the return type for the async iterator function with another special builder?

No. There's no ability in the language/compiler to substitute an implementation for any iterator, whether sync or async.

[neuecc]

@stephentoub
I was proposed this at #3629,
I'm not really convinced that they made async impossible because sync is not possible.