How to return async IEnumerable?

Question

I have the following method:

public async IEnumerable<string> GetListDriversAsync()
{
   var drives = await graphClient.Drive.Root.Children.Request().GetAsync();
        foreach (var d in drives)
            yield return d.ToString(); 
}

But compiler error says:

"The return type of an async must be void, Task or Task <T>"

How do I return IEnumerable when the method is async?

Rename your method signature to `public async Task> GetListDriversAsync()` — Kunal Mukherjee, May 07 '19 at 12:26
I tried this already, it says: "the body cannot be an iterator because Task> is not an interface type" — Codey, May 07 '19 at 12:31
The short answer is that what you want to do cannot be done in the current version of C#. You can keep your iterator method if you encapsulate it in a local function, but you will need to fully populate a collection and return this to use the `async` keyword and the `Task` return type. There is no support right now for "async yield return". — Lasse V. Karlsen, May 07 '19 at 12:34
Possible duplicate of [async Task with yield return?](https://stackoverflow.com/questions/42882078/async-taskienumerable-with-yield-return) — Frontear, May 07 '19 at 12:35
Both async/await and iterator methods involves compiler magic that rewrites your method, they cannot work together right now. — Lasse V. Karlsen, May 07 '19 at 12:35
Is C# 8 Async enumerable allowed? https://dotnetcoretutorials.com/2019/01/09/iasyncenumerable-in-c-8/ — xdtTransform, May 07 '19 at 12:35
What you are trying to do is "async enumerables" - that's a vNext feature; right now: there aren't great options for this — Marc Gravell, May 07 '19 at 12:35

score 8 · Answer 1 · answered Sep 30 '19 at 12:29

An alternative way is possible in C# 8. It uses IAsyncEnumerable.

public async IAsyncEnumerable<string> GetListDriversAsync()
{
    var drives = await graphClient.Drive.Root.Children.Request().GetAsync();
    foreach (var d in drives)
        yield return d.ToString();
}

It will alter your signature a bit, which may (or may not) be an option for you.

Usage:

await foreach (var driver in foo.GetListDriversAsync())
{
    Console.WriteLine(driver );
}

score 7 · Accepted Answer · answered May 07 '19 at 12:36

7

Try this:

public async Task<IEnumerable<string>> GetListDriversAsync()
{
    var drives = await graphClient.Drive.Root.Children.Request().GetAsync();

    IEnumerable<string> GetListDrivers()
    {
        foreach (var d in drives)
            yield return d.ToString();
    }

    return GetListDrivers();
}

answered May 07 '19 at 12:36

Enigmativity

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Just for my own understanding - why the local function and `yield return` in this case instead of `return drives.Select(d => d.ToString())` ? – Scott Hannen May 07 '19 at 13:08
1

@ScottHannen see [Local functions and exceptions](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/local-functions#local-functions-and-exceptions) which discusses exception handling with enumerators in relation to local functions – Josh Withee May 07 '19 at 15:21
@ScottHannen - The OP wanted `yield` so that's why I wrote it that way. Marathon55 is also rigth about it being the right pattern to handle exceptions. – Enigmativity May 07 '19 at 22:15

How to return async IEnumerable?

2 Answers2

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