Synchronous IAsyncEnumerable iterator method

[verdie-g]

Hello,

When trying to write a synchronous iterator method returning an IAsyncEnumerable

async IAsyncEnumerable<int> Enumerate()
{
    foreach (int i in new[] { 1, 2, 3 }) yield return i;
}

a warning is raised

[CS1998] This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.

and an error is raised by removing the async keyword

[CS8403] Method with an iterator block must be 'async' to return 'IAsyncEnumerable'

One could say use IEnumerable but I'm implementing an interface here.

Is there any blocker to allow non-async iterator methods? For example:

IAsyncEnumerable<int> Enumerate()
{
    foreach (int i in new[] { 1, 2, 3 }) yield return i;
}

[CyrusNajmabadi]

@jcouv ?

[jcouv]

This type of adaptation can already be solved via helper libraries (see IAsyncEnumerable ToAsyncEnumerable(IEnumerable)).

[verdie-g]

It can, but it sounds very cumbersome to reference a library just for that ToAsyncEnumerable method.

I guess it depends how frequent my use-case is. I personally have this issue in most of recent projects when I need to mock an interface that returns an IAsyncEnumerable.

[CyrusNajmabadi]

You can also just suppress the warning in this case with a comment explaining that this is a case where you can produce this all synchronously.

[CyrusNajmabadi]

or, you can add a dummy await Task.CompletedTask; to make things happy :)

[jnm2]

I suppress the warning for ordinary async Task methods too. It's not a bad thing to have handleable exceptions marshaled into a returned Task instance instead of throwing before the task is even awaited.