[reubenbond]

ContinueWith is fine for simple flows, but things fall apart when you need to perform multiple async operations with the same error handling. You either repeat yourself or you put the error handling code into a lambda and make sure you always pass it along.

Assuming you just want to propagate exceptions, you have to mess with TaskContinuationSource (I.e, promises) and be damn certain you have wired it into all the right places.

Async/await makes error handling much more uniform and "obviously correct" than stringing callbacks together.

[jasallen]

Exception propagation is one area I have to agree. Still, there was no reason to invent whole new language semantics for that. As far as simple flows, I'll disagree. The Task Library allows for normal workflow operations and your decisions are much more (to me) explicit when you see "WaitAll", "WaitAny" chaining things together.

[reubenbond]

"WaitAll" and "WaitAny" have their non-blocking counterparts, "WhenAll" and WhenAny", which allow you to express the same logic/structure without blocking a thread.

Nesting callbacks (ContinueWith) gets messy very quickly, and then your program structure and semantics are hidden behind masses of arcane ceremony.

Performing async calls dependent on previous async calls is hard enough to do correctly using ContinueWith, but try using it in a loop.

[contextfree]

await isn't an alternative to TPL/Tasks, it's a feature that improves a certain common use case of TPL/Tasks. "await foo" means (roughly[1]) "yield and schedule the current continuation to be run on completion of foo". What really requires language support is the "current continuation" bit, which starts to become tricky/convoluted to express directly once control flow comes in.

For any use case other than this there are other tools, such as WaitAll/WaitAny as you say.

[1] http://stackoverflow.com/questions/4070237/how-could-the-new...

[jasallen]

Noone suggested it was not built on top of Tasks.

I make it a rule never to question Eric Lippert, and I won't do so here. That said, right, the current continuation bit requires IL foo, but as the ones writing the code, there is always a way besides needing the current continuation bit.

And if you're going between languages much at all, Tasks are a lot more like promises and other paradigms in other languages. Whereas the whole await business is just... weird.