Can I await something if there's no GetAwaiter method?

Question

I saw some articles about designing a custom awaitable type:

http://books.google.com.br/books?id=1On1glEbTfIC&pg=PA83&lpg=PA83&dq=create+a+custom+awaitab

Answer 1

You can supply your own implementation of GetAwaiter via an extension method if one does not exist.

http://blogs.msdn.com/b/pfxteam/archive/2011/01/13/10115642.aspx

Answer 2

First of all, no.

You can't await something that doesn't have a GetAwaiter method that returns something that has GetResult, OnCompleted, IsCompleted and implements INotifyCompletion.

So, how are you able to await an IObservable<T>?

The compiler, on top of instance methods, also accepts GetAwaiter extensions methods. In this case Reactive Extensions's Observable provides that extension:

public static AsyncSubject<TSource> GetAwaiter<TSource>(this IObservable<TSource> source);

For example, this is how we can make strings awaitable (which compiles but obviously won't actually work):

public static Awaiter GetAwaiter(this string s)
{
    throw new NotImplementedException();
}
public abstract class Awaiter : INotifyCompletion
{
    public abstract bool IsCompleted { get; }
    public abstract void GetResult();
    public abstract void OnCompleted(Action continuation);
}

await "bar";