How does FirstAsync work?

Question

In my everlasting quest to suck less I'm trying to understand Rx.net's FirstAsync() syntax. Most documentation is for the deprecated First()
If I understand correctly it allows me to start a stream as soon as the first element from a stream arrives.

Say I have a stream myStream and I want to start a stream that takes the first element and starts a stream based on that one element. In my case it would be a stream of one.
I expect it to do this:

---1-2->
---A--->

How would I go about this?

myStream.FirstAsync().Return(() => return "A"); // doesn't compile

Answer 1

I don't know why the other two answers are saying .FirstAsync() returns a Task (or that you should call .Result). It does not return a Task, it returns an IObservable<TSource>. Observables are awaitable, but they are not tasks.

To achieve your desired functionality, do the following: myStream.FirstAsync().Select(_ => "A").

You can also do myStream.Take(1).Select(_ => "A"). The difference between this and the FirstAsync version, is that the FirstAsync version will throw an exception if myStream completes without any elements. Take(1) will complete with no error.