Rx and the Async CTP

1 Nov

With the news about C# 5.0 and the new asynchronous keywords (async and await), it comes as no surprise that the Reactive Framework team is in on this.

I was a bit dubious to the future of Rx after seeing Anders’ video, but on the day of the PDC, the Rx team released a new version of the library that allows us to use the new async compiler magic with Rx.

Since this new async CTP doesn’t rely on framework features, a new runtime or anything other than convention, all that the Rx team needed to do to have the framework work, allow the conversion of an IObservable<T> to a class that conforms to the Awaiter convention.  As the spec says:

The expression t of an await-expression await t is called the task of the await expression. The task t is required to be awaitable, which means that all of the following must hold:

  • (t).GetAwaiter() is a valid expression of type A.
  • Given an expression a of type A and an expression r of type System.Action, (a).BeginAwait(r) is a valid boolean-expression.
  • Given an expression a of type A, (a).EndAwait() is a valid expression.

A is called the awaiter type for the await expression. The GetAwaiter method is used to obtain an awaiter for the task.

The BeginAwait method is used to sign up a continuation on the awaited task. The continuation passed is the resumption delegate, which is further explained below.

The EndAwait method is used to obtain the outcome of the task once it is complete.

The method calls will be resolved syntactically, so all of GetAwaiter, BeginAwait and EndAwait can be either instance members or extension methods, or even bound dynamically, as long as the calls are valid in the context where the await expression appears. All of them are intended to be “non-blocking”; that is, not cause the calling thread to wait for a significant amount of time, e.g. for an operation to complete.


What this means to us Rx’ers, is that we can leverage the GetAwaiter() extension method provided in the Rx framework to convert between an IObservable<T> into something that works with the new async and await keywords.  This method can be found in System.Reactive.dll:

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