Asynchronous iterator Task<IEnumerable<T>>

Question

I’m trying to implement an asynchronous function that returns an iterator. The idea is the following:

    private async Task<IEnumerable<char>> TestAsync(string testString)     {         foreach (char c in testString.ToCharArray())         {             // do other work             yield return c;         }     } 

However, there is an error message that the function cannot be an iterator block because Task<IEnumerable<char>> is not an iterator interface type. Is there a solution?

Answer 1

It sounds like what you may really be looking for is something like IObservable<T>, which is sort of like a push-based asynchronous IEnumerable<T>. See Reactive Extensions, a.k.a. Rx (code licensed under MIT) (no affiliation) for a huge host of methods that work with IObservable<T> to make it work like LINQ-to-Objects and more.

The problem with IEnumerable<T> is that there's nothing that really makes the enumeration itself asynchronous. If you don't want to add a dependency on Rx (which is really what makes IObservable<T> shine), this alternative might work for you:

public async Task<IEnumerable<char>> TestAsync(string testString) {     return GetChars(testString); }  private static IEnumerable<char> GetChars(string testString) {     foreach (char c in testString.ToCharArray())     {         // do other work         yield return c;     } } 

though I'd like to point out that without knowing what's actually being done asynchronously, there may be a much better way to accomplish your goals. None of the code you posted will actually do anything asynchronously, and I don't really know if anything in // do other work is asynchronous (in which case, this isn't a solution to your underlying problem though it will make your code compile).