Async and yield Keywords in c#

Question

public static async ???? ReadFileLineByLineAsync(string file)
{

    using(StreamReader s = new StreamReader(file))
    {
        while (!s.EndOfStream)
            yield return await s.ReadLineAsync();
    }
}

I want to write a async function for reading a file line by line. what should be the return type of this function. I would appreciate any suggestions on this.

afaik, `async` and `yield` cannot be used together. Both keywords lead to creating new classes that do the async boiler-plate code (or the enumerator code respectivly), and these concepts cannot be combined (at least in current c# versions) — René Vogt, Jul 12 '18 at 07:24
@BarrJ No! This kind of question is totally off-topic on codereview — René Vogt, Jul 12 '18 at 07:25
I tried using Task> as the return. But when yield keyword is used we need a iterator interface type. — neelesh bodgal, Jul 12 '18 at 07:25
https://stackoverflow.com/questions/23295119/asynchronous-iterator-taskienumerablet — Dmitry Bychenko, Jul 12 '18 at 07:26
You can't do that because you would need to simultaneously return a `Task>` and an `IEnumerable` for this to even compile. — Tudor, Jul 12 '18 at 07:26
Here's the [language proposal](https://github.com/dotnet/csharplang/issues/43) but it's not been implemented yet — Damien_The_Unbeliever, Jul 12 '18 at 07:28

score 0 · Answer 1 · answered Jul 12 '18 at 07:27

You could try using Reactive Linq Observable instead:

public static IObservable<string> ReadFileLineByLineAsync(string file)
{
  return Observable.Create<string>(
    (obs, token) =>
      Task.Factory.StartNew(
        () =>
        {
          using (var s = new StreamReader(file))
          {
            while (!s.EndOfStream)
              obs.OnNext(s.ReadLine());
          }
          obs.OnCompleted();
        },
        token));
}

Async and yield Keywords in c#

1 Answers1