Question

I am playing with Rx in .Net3.5SP1 and trying the 101 Rx Samples. I am trying the first sample (Start – Run Code Asynchronously) but it doesn’t seem to actually run asynchronously. For example,

        Console.WriteLine("[Creating]");
        var o = Observable.Start(() => 
        {
            Console.WriteLine("Calculating..."); 
            Thread.Sleep(3000); 
            Console.WriteLine("Done."); 
        });
        Console.WriteLine("[Created]");
        Console.WriteLine("[Starting]");
        o.First();   // subscribe and wait for completion of background operation
        Console.WriteLine("[Started]");

Outputs

[Creating]
[Created]
[Starting]
Calculating...
    <...3 Second Wait...>
Done.
[Started]

Is there an explanation for this? Am I doing something wrong? Is this expected behaviour?

UPDATE

I would have thought it would have said

[Creating] 
[Created] 
[Starting] 
Calculating... 
[Started] 
    <...3 Second Wait...> 
Done. 

But the main thread is blocked while the supposedly Asynch call happens.

Answer 1

That looks reasonably expected to me.

If you put a Thread.Sleep call between “Created” and “Starting” I think you’ll see the “Calculating” line appear, showing that it’s doing work while the main thread is running. That’s the way in which it’s asynchronous.

If you’re concerned because First() returns the value itself, rather than giving a sort of “future” value you can consult later, that’s a different matter – and I have two blog posts for you to read: part 1; part 2. I think you want the Prune method, but I’m not entirely sure.